Early generation transponders had no inbuilt transmit inhibitor when changing channels therefore all pilots were instructed to switch to standby when changing.
Most modern transponders will inhibit transmission until a number of seconds after a digit has been changed. This delay is increased if the new selection is one of the emergency codes. If the a/c flight manual doesn't give any info (which most don't) check with your maintenance org or the equipment manufacturer. KNOW YOUR EQUIPMENT but if in doubt, switch to standby.
What a boring subject - suprised this generated so much discussion.