Has anyone upgraded power cord connectors?

Tough question. Sometimes audio designers choose a plug because it sounds good to them. This is not always an expensive one. Check with the guys at Audioprism first. I actually spoke to them last week at the CES in Las Vegas. They were a friendly and helpful bunch.

I have added IEC's to gear that came with a hard wired lamp cord, but have never changed just the plugs.

This IS a tough one. There's less difference on the male AC parallel spade end, but do look for a plug that has big, thick spades with large, all-brass screw-clamp contacts.
The IECs were a different story until recently. Used to be that most of us tolerated the flimsy, solder-only thin contacts in the Schurter IEC320 4700.0603, or spent $$$ on Marincos or Hubbell or Furutech.
Just recently Schurter completely redesigned their IEC320 (now called the 4781.0100) into a GREAT connector with heavy screw-clamp terminal blocks and thick spring-clamp terminals! Finally. So building affordable PCs is now easier than ever. In my opinion, however, the quality of the cable itself is overwhelmingly more important than the connector once a certain level of minimal criteria are met.
Email me for details if you want. Cheers.
Ern

Subaruguru, I beg to differ as the connectors make up at least fifty percent of the "sound" of a power inlet cord.

This is hearsay from another member, but when he replaced the chintsy plug on his AC cord the improvement was reportedly significant. Based upon my experience with the Wattgate 381 AC outlet, if replacing male AC plugs with Wattgate's product approaches anywhere near the improvement that the 381 accomplished, then you'd be a fool to not do so. But this is only an extrapolated opinion, I haven't tried that although I certainly should.