Design a database migration with a rehearsal

Design a migration for [CURRENT SCHEMA] to [TARGET SCHEMA] using [DATABASE]. Specify preconditions, backup evidence, forward statements, data backfill, validation queries, lock or downtime risks, and an explicit rollback rehearsal on a copy. Show how existing application versions behave during the transition. Return a numbered runbook and test matrix for empty, duplicate and large datasets. Do not propose destructive changes to live data without a verified backup and an owner decision. If the schema details are missing, ask for them rather than writing plausible SQL that may be wrong.