Out of interest, how old is old?
Ahhh, you noticed that I specifically dodged the issue. Who can be sure - it's down to a combination of airborne and ground equipment? For practical purposes, if your transponder does not use thermionic valves I wouldn't worry about switching to STBY unless there is some evidence/experience to suggest that it causes 'problems'. Problems may not simply be triggering emergency code alerts or duplicating code-callsign pairings in the radar system.