I have to disagree with the plumber...I think you should replace the valve. While you may have to cut the stud some, it should be doable from within the existing hole. Worse comes to worse, you could use a remodeling plate if the hole needs to be enlarged some. A remodeling plate is a larger plate to cover the bigger hole (yes, it's fairly easy to cut that material), that the new plate then fits on. I don't think that the existing valve has the required anti-scald technology in it, and I don't think it can be retrofitted without changing the valve body. While technically, just replacing the trim might not require upgrading, for your safety, and resale value down the road, it's a good idea now.
Note, unless the new cover plate extends beyond where the old one did, you'll see the discoloration where the old one stopped and be reminded of the remodeling every time you look in the shower. I don't think you can polish that out.
Think wall is something like fiberglass that is literally, thin. Your surround is not that type.