Write a Data Migration Script

Write a safe, idempotent data migration script for the following scenario:

Database: [PostgreSQL / MySQL / MongoDB / SQLite]
Language: [Python / Node.js / Go / SQL only]
What's changing: [describe the migration — e.g. adding a column / transforming data / moving to new schema / splitting a table]
Current schema: [describe or paste the relevant parts]
Target schema: [describe what it should look like after]
Data volume: [approximate rows / documents affected]
Can the app be offline during migration: [yes / no]

The script must:
1. Be idempotent — safe to run multiple times without corrupting data
2. Run in a transaction (or batch transactions with rollback on error)
3. Include a dry-run mode that shows what would change without changing it
4. Log progress for large datasets
5. Validate data before and after the migration
6. Have a rollback procedure documented in comments

Also provide:
- Pre-migration checklist
- How to verify the migration succeeded
- Estimated runtime for the given data volume