Like most bits of kit in any machine,its good to be trained to the specific kit fitted.Aeroplanes with WW2 avionics need their pilots to be trained by WW2 veterans.
If theres achance of a duff squawk from an old transponder I'm sure it should be used in a way that it won't do the wrong thing.
Modern kit eg Garmin327 only transmits a new code on pressing fourth digit
General discussion and training on say a transponder,GPS,radio etc is of limited use.I'm sure it should be type specific.
So,if you change planes,get trained on the actual kit in the new ones.In general modern avionics can dramatically reduce pilot workload when handled to the instruments designed capability and not just operated to a limited perceived capability.Put another way,its all in the manual!! .and the manual probably has lots of pages because the instrument has lots of functions over and above the ones that you thought..