The trim pieces of virtually any plumbing fixture are brand and often model specific...you can't exchange them sort of like trying to put a Chevy piston into a Ford engine.
Now, depending on how old and the type of shower valve you have, you may HAVE to change it to a newer version to meet current codes. Depends on what you have and your local inspector's view of grandfathered, verses major remodel/change. All new valves must have anti-scald features. If yours doesn't, it's time to change. If you are lucky, and you can determine the existing brand, you may be able to replace the trim pieces.
If you do decide to replace the valve, and the hole needs to be enlarged to do it (assuming you don't have access from the rear), then they do make remodel plates which will cover a bigger hole than normal.