Write a Database Migration Script

Write a database migration script for the following schema change.

Database: [POSTGRESQL / MYSQL / SQLITE / etc.]
Migration tool: [PRISMA / FLYWAY / LIQUIBASE / KNEX / CUSTOM]

Change needed:
[DESCRIBE WHAT NEEDS TO CHANGE — e.g. add a column, rename a table, change a data type, add an index, backfill data, split a table]

Requirements:
1. The UP migration — applying the change
2. The DOWN migration — rolling back safely
3. Any data migration steps (if existing data needs transforming)
4. Index considerations — what indexes to add/remove
5. Locking considerations — will this migration lock the table in production?
6. Estimated downtime (or how to make it zero-downtime)
7. How to verify the migration succeeded

Include comments explaining any non-obvious steps.