There are occasional reports of the app offering last-seat availability for SDCs. It's generally thought to be due to UA trying to clear space on an oversold flight.

Personally, I wouldn't hesitate, but my tolerance for calling up UA and expecting them to fix things if they go sideways is pretty high.

I wouldn't expect it to be clawed back, or for there to be any manual review. If something's going to go wrong, it'll likely be right away, as it's trying to reissue the ticket. If you're able to get the ticket reissued and check in for the new flight, I don't anticipate any problems.