Mrmum
It all rather depends on how long you intend to take changing the code, even my slowest students get the code set with enough time to not upset the radar system.
I have no doubt that turning the transponder to standby will totaly avoid the problem but it also leaves you invisible to traffic systems for a short time so it is time for you to do a risk assessment on this, you can:-
A/turn off the transponder to avoid the remote chance that you will trip an alarm for a second, the result of this action is the remote chance that an aircraft is near he won't get any warning and might hit you.
B/ leave the transponder on and keep some traffic protection.
On balance I will recomend option B to my students, ATC getting hacked off with me I can sort on the phone, what I can't fix is the results from a mid-air. ( no matter how unlikely).