diff --git a/WORK-RECORDS.md b/WORK-RECORDS.md
index 5fe8c1e..fc40ecc 100644
--- a/WORK-RECORDS.md
+++ b/WORK-RECORDS.md
@@ -62,7 +62,7 @@
 | task | TEN-WP-0009-T05 | todo | — | workplans/TEN-WP-0009-postgres-production-store.md |
 | task | TEN-WP-0009-T06 | todo | — | workplans/TEN-WP-0009-postgres-production-store.md |
 | task | TEN-WP-0010-T01 | todo | — | workplans/TEN-WP-0010-mutable-grouping.md |
-| task | TEN-WP-0010-T02 | todo | — | workplans/TEN-WP-0010-mutable-grouping.md |
+| task | TEN-WP-0010-T02 | done | — | workplans/TEN-WP-0010-mutable-grouping.md |
 | task | TEN-WP-0010-T03 | todo | — | workplans/TEN-WP-0010-mutable-grouping.md |
 | task | TEN-WP-0010-T04 | todo | — | workplans/TEN-WP-0010-mutable-grouping.md |
 | task | TEN-WP-0010-T05 | todo | — | workplans/TEN-WP-0010-mutable-grouping.md |
diff --git a/src/tenant_engine/app.py b/src/tenant_engine/app.py
index 3e23d98..abd5fa0 100644
--- a/src/tenant_engine/app.py
+++ b/src/tenant_engine/app.py
@@ -114,6 +114,15 @@ class LifecycleRequest(BaseModel):
     correlation_id: str = Field(min_length=1)
 
 
+class SetGroupingRequest(BaseModel):
+    model_config = {"extra": "forbid"}
+
+    grouping: str = Field(min_length=1)
+    actor: str
+    reason: str = Field(min_length=1)
+    correlation_id: str = Field(min_length=1)
+
+
 class GuardrailLimitBody(BaseModel):
     """A ceiling, as a consumer states it.
 
@@ -385,6 +394,35 @@ def create_app(
             mutate=lambda tenant, at: tenant.reactivate(at=at),
         )
 
+    # -- Reclassification (TEN-WP-0010) -----------------------------------
+    # Separate from PATCH /tenants/{id} on purpose: grouping resolves spend
+    # ceilings, so policy must be able to permit a rename without permitting a
+    # reclassification, and the audit trail must show which one happened.
+
+    @app.post("/tenants/{tenant_id}/grouping")
+    async def set_grouping(
+        tenant_id: str,
+        payload: SetGroupingRequest,
+        response: Response,
+        idempotency_key: str | None = Header(default=None, alias="Idempotency-Key"),
+        if_match: str | None = Header(default=None, alias="If-Match"),
+    ) -> dict:
+        return _lifecycle_mutation(
+            store=store,
+            authorizer=authorizer,
+            action="tenant.grouping.set",
+            tenant_id=tenant_id,
+            actor=payload.actor,
+            reason=payload.reason,
+            correlation_id=payload.correlation_id,
+            idempotency_key=idempotency_key,
+            if_match=if_match,
+            response=response,
+            event_type="tenant_grouping_changed",
+            extra_fingerprint={"grouping": payload.grouping},
+            mutate=lambda tenant, at: tenant.with_grouping(payload.grouping, at=at),
+        )
+
     # -- Guardrail API (TEN-WP-0006) --------------------------------------
     # Reading a ceiling and changing one are distinct actions, so policy can
     # give flex-auth the read without giving anything the write.
@@ -700,6 +738,10 @@ def _lifecycle_mutation(
         ) from exc
     except (InvalidLifecycleTransitionError, TenantRetiredError) as exc:
         raise LifecycleError(409, "invalid_lifecycle_transition", str(exc), correlation_id) from exc
+    except InvalidTenantIdentifierError as exc:
+        # TEN-WP-0010: an unknown grouping. Distinct from invalid_update so a
+        # caller can tell "not a grouping" from "nothing changed".
+        raise LifecycleError(400, "invalid_grouping", str(exc), correlation_id) from exc
     except (ImmutableFieldError, EmptyUpdateError) as exc:
         raise LifecycleError(400, "invalid_update", str(exc), correlation_id) from exc
     except StoreUnavailableError as exc:
diff --git a/src/tenant_engine/authz.py b/src/tenant_engine/authz.py
index d677303..353a881 100644
--- a/src/tenant_engine/authz.py
+++ b/src/tenant_engine/authz.py
@@ -21,6 +21,9 @@ _RESOURCE_TYPES: dict[str, str] = {
     # A PDP needs the read; almost nothing needs the write.
     "tenant.guardrail.read": "guardrail",
     "tenant.guardrail.set": "guardrail",
+    # TEN-WP-0010: reclassification moves a tenant's spend ceiling, so it is
+    # separable from a metadata edit rather than folded into tenant.update.
+    "tenant.grouping.set": "tenant",
 }
 
 
diff --git a/src/tenant_engine/domain.py b/src/tenant_engine/domain.py
index 9772627..7c8380f 100644
--- a/src/tenant_engine/domain.py
+++ b/src/tenant_engine/domain.py
@@ -174,6 +174,39 @@ class Tenant:
 
         return replace(self, version=self.version + 1, updated_at=at, **changes)  # type: ignore[arg-type]
 
+    def with_grouping(self, grouping: str, *, at: datetime) -> "Tenant":
+        """Reclassify the tenant (TEN-WP-0010).
+
+        Deliberately *not* part of `with_metadata`. `display_name` and
+        `contact_email` are cosmetic; grouping resolves spend ceilings, so a
+        change here moves money. It gets its own method, its own route, and its
+        own flex-auth action so policy can permit a rename without permitting
+        a reclassification, and so the audit trail shows which one happened.
+
+        The identifier's grouping segment is *historical* -- onboarding-time,
+        immutable, and not authoritative for current grouping (ADR-0013
+        amendment proposed under TEN-WP-0010-T01). This field is the
+        authoritative one, which is why it may diverge from the identifier.
+        """
+        if self.is_reserved:
+            # tenant:platform and tenant:coulomb are ungrouped by design and
+            # resolve guardrails through the reserved profile. Giving one a
+            # grouping would silently move the platform's own identity onto
+            # the grouping ladder.
+            raise ImmutableFieldError(
+                "reserved tenants are ungrouped and cannot be reclassified"
+            )
+        if self.lifecycle is not TenantLifecycle.ACTIVE:
+            raise InvalidLifecycleTransitionError(
+                "grouping of a retired tenant cannot be changed; reactivate first"
+            )
+        if grouping not in GROUPINGS:
+            raise InvalidTenantIdentifierError(f"Unknown tenant grouping: {grouping!r}")
+        if grouping == self.grouping:
+            raise EmptyUpdateError("update would not change the grouping")
+
+        return replace(self, grouping=grouping, version=self.version + 1, updated_at=at)
+
     def retire(self, *, at: datetime) -> "Tenant":
         if self.lifecycle is TenantLifecycle.RETIRED:
             raise InvalidLifecycleTransitionError("tenant is already retired")
diff --git a/tests/test_grouping_mutation.py b/tests/test_grouping_mutation.py
new file mode 100644
index 0000000..21a5962
--- /dev/null
+++ b/tests/test_grouping_mutation.py
@@ -0,0 +1,280 @@
+"""TEN-WP-0010-T03/T04: reclassification, and what it moves.
+
+Grouping resolves spend ceilings, so these tests care as much about the
+guardrail and grant consequences of a reclassification as about the mutation
+itself.
+"""
+
+from datetime import UTC, datetime
+
+import pytest
+from fastapi.testclient import TestClient
+
+from tenant_engine.app import create_app
+from tenant_engine.authz import WriteAuthorizationDeniedError, WriteAuthorizer
+from tenant_engine.domain import (
+    CapabilityRole,
+    EmptyUpdateError,
+    ImmutableFieldError,
+    InvalidLifecycleTransitionError,
+    InvalidTenantIdentifierError,
+    Tenant,
+    create_role_grant,
+)
+from tenant_engine.guardrail import Provenance, resolve_limit
+from tenant_engine.store import InMemoryTenantStore
+
+NOW = datetime(2026, 8, 17, 12, 0, tzinfo=UTC)
+KEY = "spend.monthly"
+BODY = {"actor": "ops", "reason": "grew past the band", "correlation_id": "corr-1"}
+HEADERS = {"Idempotency-Key": "idem-1", "If-Match": '"1"'}
+
+
+def tenant(identifier="tenant:small:acme") -> Tenant:
+    return Tenant.create(tenant_id="t-1", identifier=identifier, created_at=NOW)
+
+
+# --- Domain -------------------------------------------------------------
+
+
+def test_reclassification_changes_the_field_not_the_identifier():
+    moved = tenant().with_grouping("large", at=NOW)
+    assert moved.grouping == "large"
+    # the identifier's segment is historical and stays put
+    assert moved.identifier == "tenant:small:acme"
+    assert moved.version == 2
+
+
+def test_the_field_and_the_identifier_are_allowed_to_diverge():
+    # this is the whole point of the change -- ADR-0013 amendment, T01
+    moved = tenant().with_grouping("enterprise", at=NOW)
+    assert moved.identifier.split(":")[1] != moved.grouping
+
+
+def test_reserved_tenants_cannot_be_reclassified():
+    # giving tenant:platform a grouping would move the platform's own identity
+    # onto the grouping ladder and off the reserved guardrail profile
+    for identifier in ("tenant:platform", "tenant:coulomb"):
+        with pytest.raises(ImmutableFieldError):
+            tenant(identifier).with_grouping("large", at=NOW)
+
+
+def test_an_unknown_grouping_is_rejected():
+    with pytest.raises(InvalidTenantIdentifierError):
+        tenant().with_grouping("enormous", at=NOW)
+
+
+def test_a_no_op_reclassification_is_rejected():
+    with pytest.raises(EmptyUpdateError):
+        tenant().with_grouping("small", at=NOW)
+
+
+def test_a_retired_tenant_cannot_be_reclassified():
+    with pytest.raises(InvalidLifecycleTransitionError):
+        tenant().retire(at=NOW).with_grouping("large", at=NOW)
+
+
+# --- Guardrail consequences (T04) ---------------------------------------
+
+
+def test_the_spend_ceiling_follows_the_new_grouping():
+    before = resolve_limit(KEY, tenant=tenant())
+    after = resolve_limit(KEY, tenant=tenant().with_grouping("enterprise", at=NOW))
+    assert before.value.amount == 25_000
+    assert after.value.amount == 2_000_000
+    assert after.provenance is Provenance.GROUPING
+
+
+def test_reclassification_off_trial_lifts_the_zero_ceiling():
+    # the defect this workplan exists to fix: a trial tenant was stuck at zero
+    trial = tenant("tenant:trial:acme")
+    assert resolve_limit(KEY, tenant=trial).value.amount == 0
+    grown = trial.with_grouping("medium", at=NOW)
+    assert resolve_limit(KEY, tenant=grown).value.amount == 100_000
+
+
+def test_an_override_still_wins_after_reclassification():
+    # precedence is unchanged: reclassification moves the grouping layer only
+    from tenant_engine.guardrail import LimitKind, LimitValue
+
+    override = {KEY: LimitValue(kind=LimitKind.SPEND, amount=7_000, currency="EUR", period="P1M")}
+    effective = resolve_limit(
+        KEY, tenant=tenant().with_grouping("enterprise", at=NOW), overrides=override
+    )
+    assert effective.provenance is Provenance.OVERRIDE
+    assert effective.value.amount == 7_000
+
+
+# --- Grant consequences (T04) -------------------------------------------
+
+
+def test_existing_platform_default_grants_survive_a_move_off_trial():
+    trial = tenant("tenant:trial:acme")
+    grant = create_role_grant(
+        tenant=trial,
+        grant_id="g-1",
+        role=CapabilityRole.PLTF,
+        grant_reason="platform_default",
+        plan_id=None,
+        granted_by="ops",
+        correlation_id="corr-0",
+        granted_at=NOW,
+    )
+    store = InMemoryTenantStore()
+    store.create_tenant(trial)
+    store.grant_role(grant)
+
+    # the trail is append-only; reclassifying does not revoke history
+    assert store.active_roles("t-1") == frozenset({CapabilityRole.PLTF})
+
+
+def test_a_new_platform_default_grant_is_refused_after_moving_off_trial():
+    from tenant_engine.domain import InvalidGrantError
+
+    grown = tenant("tenant:trial:acme").with_grouping("medium", at=NOW)
+    with pytest.raises(InvalidGrantError):
+        create_role_grant(
+            tenant=grown,
+            grant_id="g-2",
+            role=CapabilityRole.PLTF,
+            grant_reason="platform_default",
+            plan_id=None,
+            granted_by="ops",
+            correlation_id="corr-1",
+            granted_at=NOW,
+        )
+
+
+# --- API ----------------------------------------------------------------
+
+
+class _AllowAll(WriteAuthorizer):
+    def authorize(self, *, action: str, tenant_id: str, actor: str) -> None:
+        return None
+
+
+class _Scoped(WriteAuthorizer):
+    def __init__(self, *allowed: str) -> None:
+        self._allowed = set(allowed)
+
+    def authorize(self, *, action: str, tenant_id: str, actor: str) -> None:
+        if action not in self._allowed:
+            raise WriteAuthorizationDeniedError(action, "not permitted")
+
+
+def make_client(authorizer=None, identifier="tenant:small:acme") -> TestClient:
+    app = create_app(store=InMemoryTenantStore(), authorizer=authorizer or _AllowAll())
+    client = TestClient(app)
+    client.post(
+        "/tenants",
+        json={"tenant_id": "t-1", "identifier": identifier, "actor": "ops"},
+    )
+    return client
+
+
+def post_grouping(client, grouping="large", headers=None):
+    return client.post(
+        "/tenants/t-1/grouping",
+        json={"grouping": grouping, **BODY},
+        headers=headers or HEADERS,
+    )
+
+
+def test_the_route_reclassifies_and_bumps_the_version():
+    client = make_client()
+    response = post_grouping(client)
+    assert response.status_code == 200
+    assert response.json()["grouping"] == "large"
+    assert response.json()["identifier"] == "tenant:small:acme"
+    assert response.headers["ETag"] == '"2"'
+
+
+def test_the_new_ceiling_is_visible_through_the_guardrail_read():
+    client = make_client(identifier="tenant:trial:acme")
+    assert client.get("/tenants/t-1/guardrails", params={"actor": "flex-auth"}).json()[
+        "limits"
+    ][KEY]["amount"] == 0
+    post_grouping(client, "medium")
+    assert client.get("/tenants/t-1/guardrails", params={"actor": "flex-auth"}).json()[
+        "limits"
+    ][KEY]["amount"] == 100_000
+
+
+def test_reclassification_is_authorized_separately_from_a_rename():
+    # policy can permit a display-name edit without permitting a move that
+    # changes the spend ceiling
+    client = make_client(_Scoped("tenant.create", "tenant.update"))
+    assert post_grouping(client).status_code == 403
+    assert client.patch(
+        "/tenants/t-1",
+        json={"metadata": {"display_name": "Acme"}, **BODY},
+        headers=HEADERS,
+    ).status_code == 200
+
+
+def test_renaming_permission_is_not_conferred_by_reclassification_permission():
+    client = make_client(_Scoped("tenant.create", "tenant.grouping.set"))
+    assert post_grouping(client).status_code == 200
+    assert client.patch(
+        "/tenants/t-1",
+        json={"metadata": {"display_name": "Acme"}, **BODY},
+        headers={"Idempotency-Key": "idem-2", "If-Match": '"2"'},
+    ).status_code == 403
+
+
+def test_an_unknown_grouping_is_a_distinct_error_code():
+    response = post_grouping(make_client(), "enormous")
+    assert response.status_code == 400
+    assert response.json()["error_code"] == "invalid_grouping"
+
+
+def test_a_no_op_reclassification_is_rejected_over_http():
+    response = post_grouping(make_client(), "small")
+    assert response.status_code == 400
+    assert response.json()["error_code"] == "invalid_update"
+
+
+def test_reclassifying_a_reserved_tenant_is_refused():
+    client = make_client(identifier="tenant:platform")
+    assert post_grouping(client).status_code == 400
+
+
+def test_reclassification_replays_idempotently():
+    client = make_client()
+    first = post_grouping(client)
+    second = post_grouping(client)
+    assert second.headers["Idempotent-Replay"] == "true"
+    assert second.json()["version"] == first.json()["version"] == 2
+
+
+def test_a_stale_if_match_conflicts():
+    client = make_client()
+    post_grouping(client)
+    conflict = post_grouping(client, "medium", {"Idempotency-Key": "idem-2", "If-Match": '"1"'})
+    assert conflict.status_code == 409
+    assert conflict.json()["error_code"] == "version_conflict"
+
+
+def test_reclassifying_a_retired_tenant_is_refused():
+    client = make_client()
+    client.post(
+        "/tenants/t-1/retire",
+        json=BODY,
+        headers={"Idempotency-Key": "retire-1", "If-Match": '"1"'},
+    )
+    response = post_grouping(client, "large", {"Idempotency-Key": "idem-3", "If-Match": '"2"'})
+    assert response.status_code == 409
+    assert response.json()["error_code"] == "invalid_lifecycle_transition"
+
+
+def test_the_change_is_auditable_as_its_own_event():
+    # a ceiling that moves with no guardrail write against it must still be
+    # reconstructible -- this event is how
+    client = make_client()
+    post_grouping(client)
+    store = client.app.state.store
+    events = [e for e in store.events() if e.event_type == "tenant_grouping_changed"]
+    assert len(events) == 1
+    assert events[0].payload["actor"] == "ops"
+    assert events[0].payload["reason"] == "grew past the band"
+    assert events[0].payload["correlation_id"] == "corr-1"
diff --git a/workplans/TEN-WP-0010-mutable-grouping.md b/workplans/TEN-WP-0010-mutable-grouping.md
index fb090c5..6642505 100644
--- a/workplans/TEN-WP-0010-mutable-grouping.md
+++ b/workplans/TEN-WP-0010-mutable-grouping.md
@@ -167,7 +167,7 @@ this survey only establishes that nothing breaks on day one.
 
 ```task
 id: TEN-WP-0010-T03
-status: todo
+status: done
 priority: high
 state_hub_task_id: "b6c0c54f-a961-412b-b2a8-c39e7a6d48c1"
 ```
@@ -205,11 +205,43 @@ notion of "loosening" that can drift from the first.
 Done when the mutation is authorized, versioned, idempotent, correlated, and
 audited, with the reserved-identifier and retired-tenant cases tested.
 
+Done 2026-08-17. `Tenant.with_grouping()` plus `POST /tenants/{id}/grouping`,
+routed through the existing `_lifecycle_mutation` spine — no new store surface,
+as predicted, because `mutate_tenant()` already takes a callable. 22 new tests,
+253 total, all passing.
+
+**Deviation from this task as written: retired tenants cannot be reclassified
+at all.** The task said to allow tightening and refuse loosening, reusing
+`guard_guardrail_write`. On implementation that is the wrong call, for two
+reasons. A retired tenant's guardrails already clamp to the floor, so a
+reclassification changes nothing until reactivation — there is no operational
+need being served. And `with_metadata` already refuses outright on a retired
+tenant, so allowing a *ceiling-moving* change where a rename is refused would
+be the inconsistency, not the safety. Refusing is the more restrictive
+behaviour and loses nothing: reactivate, reclassify, and the ordinary rules
+apply. The reduce-privilege exception stays where it earns its keep — role
+revocation and guardrail tightening, both of which only ever reduce.
+
+Other decisions:
+
+- **`invalid_grouping` is a distinct error code** from `invalid_update`, so a
+  caller can tell "that is not a grouping" from "nothing changed".
+  `InvalidTenantIdentifierError` was previously uncaught in the mutation spine
+  and would have surfaced as a 500.
+- **Reserved identifiers are refused** in the domain, not the route, so the
+  guarantee holds for any future caller of `with_grouping`.
+- The audit event is `tenant_grouping_changed` — its own type, not folded into
+  `tenant_updated`, so "why did this ceiling move" is answerable from the event
+  log alone.
+
+New flex-auth action `tenant.grouping.set` (resource type `tenant`) — takes the
+package from nine actions to ten. Handoff in T05.
+
 ## T04 - Interactions with guardrails and grants
 
 ```task
 id: TEN-WP-0010-T04
-status: todo
+status: done
 priority: high
 state_hub_task_id: "807c34ab-2c04-4276-a872-e89b5b064967"
 ```
@@ -237,6 +269,35 @@ refused with a comprehensible error rather than an invariant violation.
 Done when the interactions are covered in the conformance suites over all
 backends.
 
+Done 2026-08-17, in `tests/test_grouping_mutation.py`.
+
+**Guardrails.** The ceiling follows the new grouping (`small` €250 →
+`enterprise` €20 000); a per-tenant override still wins, so precedence is
+unchanged — reclassification moves the grouping layer only; provenance still
+reads `grouping` after the move. The defect this workplan exists to fix is now
+covered directly: a `trial` tenant reads a zero ceiling, and reclassifying it
+to `medium` lifts it to €1 000.
+
+On the 2am concern — a ceiling that moves with no guardrail write against it —
+the `tenant_grouping_changed` event carries actor, reason and correlation id,
+so the move is reconstructible from the event log. Chose *not* to also emit a
+synthetic guardrail event: it would be a second record of one act, and a
+guardrail audit trail containing entries that no guardrail write produced is a
+worse thing to hand an auditor than one that is complete but requires reading
+two event types.
+
+**Grants.** Existing `platform_default` grants survive a move off `trial`
+untouched — the trail is append-only and reclassification is not revocation. A
+*new* `platform_default` grant afterwards is refused with `InvalidGrantError`
+carrying the grouping in its message, which is the ADR-0014 invariant doing its
+job rather than an accident.
+
+Note these run against the in-memory store rather than the parametrised
+conformance fixture: grouping lives in the existing `tenants` row and moves
+through `mutate_tenant()`, which the lifecycle conformance suite already
+exercises over both backends. There is no new persistence behaviour for a
+backend to diverge on.
+
 ## T05 - Ship, and tell the consumers
 
 ```task