How would you deploy a database change while old and new application versions run together?

Use an expand-and-contract sequence. First add compatible schema elements, then deploy code that tolerates both representations and backfill data in controlled batches. Verify completeness and behavior before switching reads or enforcing new constraints. Remove obsolete fields only after old application versions and background jobs no longer depend on them. Consider locking, replication lag, rollback, and data changes that cannot simply be undone. A migration that works on an empty local database can still interrupt production if it rewrites a large table under a long-held lock.