My apologies for the confusion. There's nothing "pedantic SOB'ish" about correcting someone who's wrong! (The first time I was ever offered a same day flight change, the AAgent described it as a "confirmed standby" and that's what I've called it ever since.) I stand corrected. Thanks to all who've pointed it out.

I'm delighted and frustrated to hear that same day flight changes retain the upgrade priority of the original booking. I'm currently sitting at LaGuardia four hours before my flight to O'Hare, and every flight is so oversold that none of my standbys are clearing. This morning I had the option to make a same day change to the flight that just departed, but I declined to do so as I didn't want to nix my upgrade chances. Fool!