As an EXP, I've had two upgrades on partners. I don't fly partners a lot though, so it is actually a high percentage:
1. BA LHR-GOT, Y appeared oversold (I think there was a canceled flight the day before) and I was bumped to J about an hour before departure.
2. CX MNL-HKG, booked W but bumped to J when A350 turned into A330 with no W cabin a few weeks prior to departure. CX reissued the ticket into J, so when it turned back into an A350 a few days prior to departure the bump stuck.
Both were really op-ups. Fun, unexpected, and nothing I'd count on happening.