From an ATC's point of view, pilots SHOULD select transponder to standby prior to changing the assigned code!
The reason is that in the process of changing the code, the radar may sweep and pick up an intermediate code which has been assigned to another aircraft. Alternatively a wrong code could be selected, and be transmitted prior to the error being realised by the pilot. This could cause an erroneous code/callsign coupling. This is particularly a problem with the newer ATS systems which display flight plan, ADS and radar tracks, as a pilot squawking the wrong code can cause the deletion of the aircraft symbol of a lower hierarchy (eg: a flight plan track).
Of course, transponders and radar systems do have features to mitigate these risks, but they don't seem to work...