Clone Server A -> Server B. All code still refers to A.
Update code to refer to B
You can now EOL Server A, and B becomes the new canonical reference.
I think the key is simply that you can keep both A + B running during the migration, so you just need to be able to do a code freeze for the duration of the migration. And a single person can easily do this migration with a few python scripts and an hour.
Bonus points: code freeze guarantees your #2 - no changes to how the code works.
Of course, I'd assume that most complaints come from situations where this "secret trick" isn't viable
Clone Server A -> Server B. All code still refers to A.
Update code to refer to B
You can now EOL Server A, and B becomes the new canonical reference.
I think the key is simply that you can keep both A + B running during the migration, so you just need to be able to do a code freeze for the duration of the migration. And a single person can easily do this migration with a few python scripts and an hour.
Bonus points: code freeze guarantees your #2 - no changes to how the code works.
Of course, I'd assume that most complaints come from situations where this "secret trick" isn't viable