Replacing only the top half of a thermostatically controlled radiator valve?

Rather than having to drain & then bleed etc the system is it practical to replace only the top of of a thermostatically controlled radiator valve. The original (PEGLER Terrier) is probably 10 to 15 years old so it will be a different top. If the threaded metal colar & the plastic body are the same fit it should be OK shouldn't it?