Originally Posted by
docbert
Upgraded passengers (regardless of the form of upgrade) are supposed to be rebooked in their original class of service, even when being rebooked onto another United flight. The exception would be where they schedule a new flight as a replacement (eg, UA839 -> UA1745 the next day) in which case upgrades should stick.
Occasionally there are reports of people being rebooked onto a different airline in their upgraded class of service, but they are rare and probably simply come down to human error when looking at the booked class.
I don't believe rebooking into an upgraded cabin routinely happens. I had upgraded flights (B fares) canceled two days in a row for a flight to FRA during the alleged ORD snow event in early January. I ended up needing to fork over 2700 to get into a front cabin despite availability of seats in C the next day.