Changing out bath and shower hardware

I want to change out all the tub/shower hardware just for cosmetics - change from brass to chrome. Is this suitable for a novice? And do I need to cut into the wall behind the tub, or can it all be done from the front? In one room this means going through a closet, but in another it is the master bedroom wall.

You shouldn't have any problems with doing that, you will just be restricted to the same manufacturer and possibly style depending on the manufacturer. It's only trim and escutcheons and such, you could take a photo of your current set up and tell a plumbing wholesaler whether it's moen/delta/american standard/what-have-you and they could let you know which valve that trim goes with. From there they could show you what other trim options match that valve. The only tools you would require is a multi driver and an allen key set, no holes would need to go anywhere...

Kristi pretty well summed it up. Faucet handles and trim are not generic. You must get parts made for your brand and model.

If if is a major brand, and not TOO old, you will be able to find new trim with some options on finish and style. Moen and Price Pfister particularly are easy to update. Other brands, more or less.