I want to swap a K1 radiator with a P+, the same size on a combi system. Can i turn off both valves, remove the existing radiator, put the new one on the existing brackets, and reconnect the existing valves, and then top up. Would draining the entire system be necessary? If draining is not necessary would i have to add inhibitor to the system if i just replaced the radiator as i have suggested. Obviously from the question you can see i am a diyer.