Originally Posted by WIRunner

I've been able to successfully upgrade day of on a UA flight purchased on a US itn. It has always been at the gate, and right before boarding though. Can never seem to get added to the list any sooner than a few hours before the flight. So if you're on an elite heavy route this might not work well. (Ironically the ones that have always cleared have been the mid-day LGA-ORD flights, but never could on ORD-ATW)