To standby or not?
In the UK, It's not a myth that need busting. Modern transponders are perfectly okay to change code without placing in standby. However the old rotary dial through ones if not placed in standby do cause alarms to go off. In D&D every time someone dials through one of the emergency codes alarms do in fact go off and the "emergency" squawk flashes up on a screen. If controllers are already dealing with other emergencies, that false "emergency" distracts them from what they are doing.
As for Swanwick controllers having problems with squawks changing, of course they don't as I can't imagine there are too many airliners around with the old dial type transponders plus virtually all the Swanwick CAT is operating on ORCAM squawks and therefore retain the same squawk throughout.
If you have an old dial transponder, switch to standby.
Moli