I'm not one to chase hotel upgrades and the vast majority of my North American stays are at hotels with slim-to-no upgrade options so it's far from a scientific study -- but I have gotten more than a handful of upgrades after app check in, with that number increasing in frequency over the past 18 months (I think a combination of hotels generally figuring out what to do with OLCI and perhaps some nudging from corporate that not consistently upgrading app using travelers was serving as a disincentive?)

Depending on ?things? it shows up as something like "We've got a great new room for you -- and it's ready!" in the app with the 'new' digital key or a "Digital key unavailable - please see the front desk when you arrive" with no digital key at least until you've said hi to the FD.

Of course both of those happen under other circumstances, including the former at one property (the HWS RIC airport...stay immediately preceding the 'party stay' I posted about for those keeping tally) where they were trying to sneak downgrade me from the room type I booked and specific room I ECIed to, or if they have to make a lateral move for some reason.

If the upgrade is that important, though, I probably would not OLCI or if I did OLCI check at the FD. One thing to remember, at least in pre-COVID times, nicer properties would, IME, do something 'nice' to the room you were assigned (via OLCI or front desk lottery) -- like candies/chocolates/flowers in the room, personalized note from mgmt, etc. -- which may make a room change after it's been assigned less likely.