My house is approximately 20 years old and the current fixtures in the bathrooms are Price Pfister. They are the plastic knobs that pull on and turn right or left to adjust the water temperature. We have had continuous problems with the knob being very difficult to pull on, and we have replaced the cartridge 2 or 3 times with some success for a short period. I decided to update the fixtures and found a Price Pfister fixture that is a handle that turns to turn the water on and adjust the temperature. I was hoping that it would be as simple as changing out the cartridge and hooking up the new fixture. I was surprised to find the entire valve in the box when I hooked it up. The hole in the shower wall is 2" or less so I cannot see what is behind the wall. There appears to be two parts to the valve; the brass part that attaches to the water pipes and shower and tub pipes, and the part that sticks out the wall and holds the cartridge (it is screwed into the brass piece by four screws. I understand that if I am going from a "pull and turn" fixture to a "turn" fixture that it won't work, but is there any way that it could be as simple as unscrewing the old piece off the valve and screwing on the new piece that holds the cartridge?