Stop by your local Home Depot book section and checkout the Plumbing 1-2-3 book. I think they cost $20, but if you have a photographic memory, It's free. Checkout your piping first see if it is copper, galvanized, pex,... there could be an access panel in the wall behind the shower. While at Home Depot ask the associate to get you all the parts and have him explain how to do the job. Hope that helps, Dan Martyn

I had a similar situation about a year ago, but I don't know much about plumbing, so I called a plumber. It seemed to me that this is the best option, but I was wrong. The plumber demanded a very large sum for his services, which was a great surprise to me. I didn't have the amount he wanted at the time and I refused his help. I decided that I'd rather save money and try to do it myself. Fortunately, there is a lot of useful information on the Internet that eventually helped me. I found all the necessary information about the valves at https://anixusa.com/gate-valve and it helped me find the right valve for my pipes. After a few hours, I managed to change it and everything was functioning quite well. From now on I fix my own plumbing at home

You’ll need a plumber for this, and they will easily tell you what you can change it out with and do it with ease.

I am not a professional but it seems like a penny wise/pound foolish decision not to replace as much as possible when you are doing a major renovation. Everything has a useful life so at some point I would assume you would need to replace. I remodeled relatively recently but prior to that I had to replace a cartridge and it was an expensive plumbing job. As I recall I also felt lucky that the plumber was able to find something that was compatible - ugly but compatible :-). Also - although you didn't mention it, since you are complete remodeling anyway, you wouldn't be limited to Moen fixtures that fit the existing plumbing.