We are replacing an acrylic shower unit with wonderboard/kerdi and ceramic subway tile. The unit doesn't go all the way to the ceiling and the walls of it are 1 1/2" thick. The shower handle is of course mounted on the acrylic shower wall, but the faucet is mounted through the drywall above the top of the shower unit.

The 1/2" wonderboard will match up with the surrounding drywall (probably doing to the ceiling with the board and kerdi, but the drywall extends out past the face of the shower now). Then there is the thinset, kerdi, thinset, and 1/4" tiles on top (so about 1/2" I think). So the new finished surface will be about an inch back from where it is now where the handle is, and about 1/2" thicker where the shower head is. Does that make any sense? It just barely does even in my head... :

The faucet is just a builder-grade , single handle Moen. I am replacing it with another single handle Moen. Can I get away with just putting the new ones on, or will the valves stems be too long/short? Or is there some other problem I'll have that I haven't even thought of? Or is it good to go ahead and replace the valves while the wall is open? It was put in in 2003. Thank you for any help.