Not an upgrade issue, but one should be careful when they split:

My companion and I were flying ORD-PHX-ORD on the same PNR. I used united.com, intending to change just my own return leg to be AUS-ORD. Somehow I ended up changing companion instead (turns out this mistake is not hard to do -- this split feature should probably have been engineered better by the developers). Phone agent changed both flights back for me, and charged a new fare difference on both ends but waived change fees (~$150 total to put it back the way it was).

Oops.