The Plat

On a serious tone, answer with my logic : if I have to prepare in advance, I would op-up the guy who didn't check in. If he doesn't show up, I don't need to upgrade anyone.
Now, if I know by statistics that I will have to offload someone, I will of course offload the guy who didn't check in.

This said, in real life, it doesn't work this way.

They will wait until the pax-list is complete, and cherry-pick among confirmed pax.

Two years ago, I was op-uped at airport check-in. I assume they were 200% certain that they will have to upgrade some people. Unsurprisingly, upgrade calls continued once at the lounge, i.e. once the pax-list was frozen.