BpmnMigrationPlan : A validated, fully-resolved rewrite: every identifier substitution needed to move an instance’s persisted state from the source version’s vocabulary to the target’s. Only entries that actually change are present, so an empty plan with equal process ids is exactly an identity migration. A plan is necessary and not sufficient. Three things stand outside it: ActivityState.external_worker and RetryState.max_attempts are dispatch-time snapshots, frozen by decision; history[] records what actually executed under the ids it executed under, immutable by decision; and the eight uuid_v5 derived key formats that embed element and flow ids are not yet handled – every key of elements is an element whose derived child-job ids change, which the engine-side applier must re-key.