There's only water pressure at the diverter when you have the valve open, so you shouldn't have to turn it off. As to what is required to repair it, not sure. Parts are available for most Delta stuff. There may be some water in the pipe from the valve to the showerhead, but it would drain out and stop once you opened things up.
If you're in the market to update the valve to one meeting the current anti-scald code, then you'd have to replace the whole valve. If you don't have room from behind to access it for replacment, then you'd have to cut the hole around the valve larger (there's a template in the remodel plate designed to cover the bigger hole). You'd have the choice of getting another valve with a built-in diverter or using one incoprorated into the tub spout. A little caulk can be used to seal around the spout.