So your question is - can you replace the trim using your existing valve? You need to know the manufacturer. Newer Moen & Kohler and even many older Grohe each use a valve that can take many trims (but -- all from the same manufacturer -- you can't use a Kohler trim on say a Moen valve or Grohe with Kohler say). When I walk through the plumbing aisles at Lowes and Home Depot I see a lot of replacement parts that are supposed to be universal, but I don't know that I'd trust those parts for longevity. You might be surprised what a good plumber can accomplish without lots of demo. Sounds like you're retiling the baths -- then replacing the valve vs trim shouldn't be a big thing. Good luck!

The trim plate is sized to cover the hole in the wall. The hole in the wall is sized to allow for replacement of or maintenance on the valve without having to demo any additional wall tile. You could go with a different sized hole or take a single large hole and turn it into two smaller ones. Should maintenance or replacement ever be required, you can go through the tile or go through the back side of the wall if access can be had that way. Just watch what you mix and match. Some escutcheon plates are simply held in place by the knob, some by screws that screw into the valve body. Some have small notches that register them to the valve body. If you decide to go out on your own, just watch for those details.

We're finalizing some selections for our bath remodel, and will be primarily using Grohe - Freehander shower, Arden valve trims and corresponding valves. We'll also be installing a hand shower. I'm wondering, if I use a Grohe valve, do I have to use a Grohe hand shower, or can I use another brand? As I understand it, the valve and valve trim are the only components that are directly connected (literally and figuratively). Thanks!

Code requires a shutoff valve for all fixtures "Except" a tub or shower, however that is to be considered "Code Minimum". You may install any number of additional shutoff valves at your discretion.