Some pictures of your existing pipes would help. Since the walls will be open, it might be a good idea to update the plumbing depending upon what's there.

I guess I wasn't clear, we are not opening the walls around the tub and that is why I was told we could not change the fixtures.

The shower head "should" unscrew from a fitting in the wall. Depending upon what you have for a faucet, the trim might be changeable. Same for the spout. Pics would help.

I think there is a good chance you will need to replace the valve inside the wall. If the tub/shower is tiled, can you get into the wall from the backside?

i'm in the same boat, and finding we cannot replace our trim without changing the valve as well.

Access it from the back side to change things out. And be prepared to be horrified at the water penetration that you find and have to remove the tile and tub anyway.