You might get lucky if you are trying to replace a Moen faucet with another Moen faucet, but lucky is the key word. When you remove the large round plate on the front (F-in your illustration) you will see how much access you have to the valve.
The shower arm (part H) and the shower head (part I) are threaded on, there should be no screws. You may find a small flat section on the arm, which will allow you to put a wrench on. Pad the 'teeth' of the wrench with sturdy tape or something similar so you don't mar the finish on the new fixture. But often, you don't even need the wrench, just a little muscle and a tight grip.