Covers changing schema or data shape without downtime using expand-then-contract — adding new structures alongside old, backfilling historical data in batches, dual-writing during the transition, verifying both old and new code paths, and keeping every step independently reversible. Use this whenever the user renames a column, changes a data type, moves data between tables, backfills a new field, or asks how to migrate without a maintenance window. For deploy sequencing of each phase use `deployment-strategies`, and for day-to-day database operations use `database-operations`.
