Database Migration Script Generator

Generate a database migration script for the following change:

Current Schema:
```sql
[PASTE CURRENT SCHEMA OR DESCRIBE IT]
```

Desired Change:
[DESCRIBE THE SCHEMA CHANGE NEEDED]

Database: [PostgreSQL/MySQL/SQL Server]
Migration Tool: [Prisma/Knex/TypeORM/Flyway/Raw SQL]
Data Preservation: [YES/NO - existing data considerations]

Please provide:
1. Up migration script
2. Down migration script (rollback)
3. Data migration steps if needed
4. Pre-migration validation checks
5. Post-migration verification queries
6. Estimated migration time and locking considerations