Can I change a radiator valve without draining the system?

Hi, I have an old gravity hot water pumped heating system, the boiler is a reliable, but expensive, Vulcan Continental but there is a lot of noise/air in the radiators/system that I think has come from a leaky radiator valve in the hall.
I have a new valve to fit but can I do it without draining the system? Could I close all the valves on the remaining rads and just drain the faulty rad or am I just wasting my time?
The system only has one cold water storage tank in the loft and not a separate one for the central heating.
The hot water tank has an immersion heater for summer use.

Any advice would be greatly appreciated

Andrew Smith
November 2007