If your plane is a certificated, production model, and not an amatuer-
built experimental, then only an avionics shop that is an FAA-
certificated repair station can legally repair or replace a
transponder system, according to my local FSDO (Houston, TX). Even
though there are slide-in replacement units available from Honeywell/
Bendix-King and Narco that would slide right into your existing
"legacy" KT-76 tray, it is not legal for the owner/pilot to swap out a
transponder as "preventive maintenance" like it is legal to do so with
a slide-in nav/com radio... all because the transponder and altitude
encoder installation must be calibrated/certified when installed or
whenever any repair work is done on them, and also every two years to
ensure they're working accurately.

The builder of a homebuilt experimental plane such as a Van's RV
series can install and/or replace his own transponder and encoder, but
cannot legally power it on in flight until he first gets it inspected
and certified by a certificated repair station avionics shop, in fact
the builder is technically legally supposed to placard the unit
"INOPERATIVE" until the certification is done and if he builds the
aircraft at one airport under a class B "mode-C ring" like in the
Houston area and has to fly to another airport to get the transponder
certified, he's supposed to call up ATC in advance to get special
permission to make the flight without the transponder. That's what a
friend of mine had to do after he built his RV-6. Calling ATC and
obtaining advance permission to operate transponder-less for the
flight to the avionics shop airport was no hassle at all.