In short: nothing.
If you rename these files so they follow Plex’s required naming schema, chances are big they will be marked as ‘newly added’ - even on the old server.
If you created a copy of these files, renamed them properly and added them to the old box, they will probably be recognised as the right movie, but won’t inherit the watched status and ‘added at’ date from their misnamed counterparts.
Either way, as soon as you have ‘unmatched’ items, you will get major trouble when moving servers.
The only chance would be to edit the database directly:
But that is a lot of effort.
