I had a slightly frustrating experience trying to use the "Upgrade Reservation Using Miles" function yesterday... I was trying to upgrade the return segments on an IND-IAH-LAX/SAN-IAH-IND itinerary for both my wife and myself, on two separate PNRs. I went through and upgraded my wife's PNR first, then my own. In each case, it showed that both return segments were available for upgrade, deducted the miles, and then showed that both segments had been successfully upgraded. However, when I went back in to look at the reservations, only the SAN-IAH segments were in First, not the IAH-IND segments.

I called the Elite line, who forwarded me to the Electronic Support Desk. Initially the rep told me that this happened because only one upgrade seat was available on the IAH-IND flight, but that didn't make sense, because neither of us were successfully upgraded. She then said she could go ahead and upgrade one of us, but not the other. Eventually, after some more complaining on my part, she somehow found a way to upgrade both of us.

So all's well that ends well, but it seems like there is a bug in the website if it shows availability and then says it has confirmed an upgrade, when it really hasn't.