Phil I ran a 53/39 x 12/27 and would switch when I could be bothered to 50/34 x 12/25. The chain was good for both. Although it is best to change the chain when you change the sprocket and keep them as a set. This way if one of the chains are worn you will only have one worn cassette.
If you are running Hollowtech II you would be better using the special tool although it is not necessary on SRAM you do not need a special tool as the crank has a a self extracting system.
I change both systems on different bikes and with front mech adjustment can have either one done on under 5 mins.