Going to float two possibilities for upgrades that I don't think have been mentioned (though am late to the party on this thread):
1. If a prior guest in a room failed a temp check/otherwise reported symptoms of Covid, or worse tested positive while at the resort, they probably have some sort of a protocol for putting that room out of commission for a while.
2. I read that during low occupancy periods post-COVID-19 when large percentages of rooms were going empty due to lack of demand, to facilitate houskeeping/keep HVAC costs down, they were trying to consolidate guests into certain buildings and leaving other buildings empty. This may result in an upgrade if they ran out of smaller rooms in the buildings they were keeping open. Obviously, I think we are now past the point where that is a thing, but may still be true at SSR where it seems there are still plenty of rooms in all/most categories this summer.
3. Cash guests who had booked at resorts that were not yet open were typically moved to the most comparable resort--which was often the
DVC side (e.g., Beach Club guests moved to BCV, Wilderness Lodge to CCV/BRV, Poly to PVB or GF). In some cases, those guests were upgraded to a suite, but some of those hotels had 1BR suites that only slept 4 (I know dvc have exceptions--but they aren't going to ask a cash guest to bring an air mattress) and studios that slept 5, so a family of 5 might need a studio, prompting then to bump up a DVC guest with from a studio to suite.