My company helps companies do migrations using LLM agents and rigid validations, and it is not a surprising goal. Of course most projects are not as clean as a compiler is in terms of their inputs and outputs, but our pitch to customers is that we aim to do bug-for-bug compatible migrations.

Porting a project from PHP7 to PHP8, you'd want the exact same SQL statements to be sent to the server for your test suite, or at least be able to explain the differences. Porting AngularJS to Vue, you'd want the same backend requests, etc..