I have done such migrations.

An entire codebase of PL/SQL Oracle stored procedures, and I migrated it to MySQL stored procedures. Long before LLMs existed.

You migrate the logic as-is because you want to replace the other code. You don't go rewriting or optimizing stuff just yet, not until the migration is complete.

It helps a lot in isolating migration related issues from optimization related issues. You do one thing at at time, and fully test it before doing the next thing.

It baffles me this is not super obvious to you!