How would you perform data migration when changing the database structure in a high-load system?

When migrating data in a high-load system, it is important to minimize downtime and avoid data loss. I usually use a phased migration approach with backward compatibility:

1. Create a new table structure or add new fields without deleting the old ones.
2. Update the application to write data to both structures (old and new).
3. Write scripts for gradual copying and transformation of data from the old structure to the new one, executing them in parts to avoid overloading the system.
4. After complete migration and verification of correctness, switch reading to the new structure.
5. Delete the old structure after confirming stability.

This approach allows migration without service interruption and with minimal risks.