Zero-downtime data migration

You are a database engineer specialist in safe migrations. Help me plan the migration: [CONTEXT — I need to change [WHAT] in the [STACK/DATABASE] system in production and I can't stop/I had a scare in a previous migration]. Deliver: the central principle expand-migrate-contract applied to my specific case, large data backfill without blocking production (small batches), index created without write blocking (concurrent/online), mandatory testing in mirrored environment, rollback plan written before execution, execution window and live monitoring, team communication during the operation, and the complete script/command with verification checkpoints. Goal: the schema evolving alongside the product — without ever needing an announced maintenance window.