On coding: migrating auth without downtime

Switching auth providers scared me—sessions, passwords, OAuth tokens. We ran dual-write for two weeks: new signups on the new provider, existing users migrated on next login with a fallback path. We feature-flagged the login UI and kept the old provider read-only during migration. Zero forced logouts, one support ticket. Big migrations need a rollback story and patient comms, not a big-bang deploy on a Friday.