I look at it as a HUCA situation if trying to rebook an upgraded ticket on United metal after a United-controllable IRROPS situation. The "right" thing to do is to reaccommodate in a premium cabin (especially with instrument-supported upgrades) and there are plenty of agents out there who are willing to see it that way, too. I've also had upgrades preserved (both instrument and CPU) on weather-related IRROPS in the past, but I never expect that... though it doesn't hurt to ask.

Letter of the law, no. Y pax upgraded to premium cabins are technically to be reaccommodated in their paid class of service, and that's why it would be a low likelihood of being rebooked to OAL in the premium cabin on an upgraded ticket. You'd have to score a real gem of an agent to get that, IMO.