Plan a schema transition that survives rollback and re-upgrade.

A fictional PostgreSQL service is renaming customer_name to display_name without changing its meaning. NULL is a legitimate name value. Version v0 reads and writes only customer_name. All application and batch writers are controlled by the team. Old and new instances overlap during deployment; product requires a two-week rollback window. The proposed migration immediately renames the column.

Design compatible version v1 and later new-only version v2. Trace this row: old=Maya, new=Maya after backfill; a rollback to v0 changes old to Mira while new remains Maya. Explain how a later upgrade returns Mira. Include a concurrent write during reconciliation and state who or what prevents an unsafe version from receiving traffic.