It can be a fair amount of work. You have to cut a large hole in the wall from either the front or the back, cut out the existing plumbing valves, solder the new ones in place, and repair the wall. If you only have access to the interior of the wall from the shower side, you will have to judge whether the destruction you need to do to the wall is worth the effort. If you are only concerned about reducing the volume of water at times, it would be far simpler to simply install an in-line valve at the shower head. It would cost less than $20, take 5 minutes, and allow you to avoid tearing into the wall.

Here is a link that might be useful: Shower volume control valve

This post was edited by kudzu9 on Wed, Dec 10, 14 at 18:28