It depends. Some if the Garmin xponders are "will fit" replacements for
the Narco 50-155 series. With their (Garmin's) STC, a logbook entry and
transponder check is all that is required. If wiring is involved , a 337
is needed.

Rutger wrote:

Whenever someone replaces an existing transponder with a different
brand or model, is all that will be needed paperwork-wise just a
logbook entry, W&B update and a new 24-month transponder certification
test done? Or is this treated really obtusely and require approval of
a 337 to describe the removal of the existing transponder plus another
337 approved and filed to describe the installation of the new one in
addition to the logbook, W&B?