Database Migration Phased Rollout

Break down this database migration into safe, reversible phases:

Migration Scope:
{{migration_scope}}

Current Schema:
{{current_schema}}

Target Schema:
{{target_schema}}

Constraints:
- {{downtime_tolerance}}
- Data volume: {{data_volume}}

Decompose into phases:

1. PRE-MIGRATION PREPARATION
   - Schema compatibility checks
   - Backup and recovery testing
   - Rollback script validation

2. SCHEMA EVOLUTION STEPS
   - Additive changes first
   - Dual-write implementation
   - Data backfill procedures

3. APPLICATION CUTOVER
   - Feature flag coordination
   - Read path migration
   - Write path migration

4. CLEANUP PHASE
   - Legacy column removal
   - Index optimization
   - Constraint enforcement

For each phase, specify:
- Execution steps with exact commands
- Verification queries
- Rollback procedure
- Success criteria
- Estimated duration