Changing coin vents on radiators- can I just shut the valve that is on the radiator, - at bottom, instead of .?

My house is heating with gas. I have several radiators that are leaking at the coin vents. they , the vents need to be changed. I noticed on the bottom of the radiators there are knobs, can i just shut the knobs when changing the coin vents OR do I have to drain the whole system and refill the boiler?? I am getting conflicting answers. thanks