Bingo, I think it takes a while for the seat to show available after someone moves out of it. I suspect this is either 1) by design to allow for an "undo" option if someone gets moved by mistake (unlikely) or 2) a by product of the massiveness and age of UA's systems (likely).

I think you have a pretty decent chance of things staying as expected since you have a boarding pass. While I don't think it's unheard of for seats to randomly change after check-it I expect it a lot less likely than changes that randomly occur weeks or months before a flight.