Lead
1) "NEW" is key word - Point to your transponder, is it new? I doubt if it is a KT76A, at best newly overhauled. But that is not NEW.
2a) System is Key word - I don't recall reading that in context of this subject (but could be wrong) A replacement KT76A with different S/N is not any more than a component change.
2B) a complete change of system with a different model transponder will require it to be Mode S compliant - Installation being key word.
Yes endless amounts of repairs can be made to current mode C transponders.
Why use the avionics shop for LRU with self test function?