In nearly all of the retros I've done I rework the supply and returns which normally lie outside the 5-6" hole that the valve comes through. If you have access to the back of this wall you may not need to mess with the tile.
Hopefully you are lucky and you have a closet directly behind the tub...if you do, get the larger Oakly plastic access panel from Lowes or Home Depot and cut yourself the access you need through the drywall. You can then install the panel with two screws & you'll have permanent access to the area without bothering with your shower tile. We use these alot and you can paint them to match the wall.
Just like KMS said. You will need to open the wall to do the plumbing. The size of this demo would be determined by the quality of the plumber and his or her experience in doing this type of work.
There are several custom retrofit covers with over sized plates that can be used when doing this type of valve change. Check with the plumbing and bath supply store, your not going to find this at your local big box store.
Hmm well there is no access panel so it sounds to me like I have to stick with a Moen posi temp assembly. Thanks everyone.
If the valve is leaking or not working well, just a "cartridge" swap will usually do the trick. Swapping out the valve "body" would be just as invasive as a complete new install
What's behind the shower?
KMS at the moment the shower assembly is working fine although very worn. The bathroom faucet and vanity top has some serious wear an tear and I'm going to change out the top and faucet. The shower and tub assembly is worn but works. Since I was going to replace and update the vanity area I wanted to go from the old "GOLD" faucets to brass in the shower area also. To keep it matching
HandyAndY the Garage.
Baby H, If the garage is behind the shower area it is not difficult to cut out and replace the Sheetrock on that area of the wall. Unless of course you have all sorts of cabinets etc on the wall. Have you thought about doing it this way? Also you can install a access panel when removing the wall board that you need to gain access. This will allow you in the future to regain access to that area should you ever need to address the faucet issue again.
You might be able to get a "new" knob and trim kit to make it look new. Swapping out a spout is pretty easy too.
KMS I went to their website once I got a few questions thrown at me asking about what type it was etc. So supposedly if you get the same type you have installed but just a different style/color I should be fine. I will see. Woodbridge it is in the cuby whole area where there are a few shelves so I won't want to go that way unless one day truly breaks/leaks