So to put all the technical information above into context there is no need to set stby when changing transponder codes, in fact it is a disadvantage to do so.
So another bit of aviation folk lore is busted ! It makes you wonder who makes up this stuff , how much more utter rubbish is practiced in the name of airmanship ? And how much cockpit time is spent away from important tasks like lookout just to keep black art brigade happy?