The difference I see is that pretty much every A&P mechanic has changed a fuel selector or done the sort of maintenance actions you described. Doing it on a different airplane is still back to the basic skill/knowledge set any A&P should have, and the minor differences are covered in the maintenance manual for that airplane. OTOH, doing an IFR certification on a GPS (including the installation and testing) is not necessarily something that the average local A&P has ever done or seen done, and probably isn't covered in 43.13-1B or the A&P training course at most schools, and so I would think should not be done without supervision or approval of the finished work if the A&P has never done it or seen it done before. If that's a misunderstanding on my part, please chalk it up to unfamiliarity with the details of the rules covering A&P's.
Personally, I would want this done by a certified avionics shop with experience in the process, and that's how I've done it on the three airplanes which I've had equipped with IFR GPS's. I also did the flight tests for that certification with a shop technician who'd flown lots of these before riding shotgun -- they even had a canned flight test profile set up in cooperation with the Airworthiness folks at the local FSDO which monitors their shop. All in all, the intricacies of the process seem to me to demand a specialist, but that's a personal decision about which others may choose differently.