I concur with e_c.
77xx (where xx<>00) does not set off the alarms at ATC units for miles around. So don't worry too much about that. If you don't want to go to standby, set the least significant digits first, thus enduring you don't go through 7700 or whatever just as it's being interrogated.
On more advanced transponders (not the silly little things you get in a PA28 from your flying club) the transponder will put itself into standby for a few seconds the instant one of the digits is changed.