By way of anecdote, I was on a United flight today which had a last minute no show in First. The flight attendant came on board, and handed the lady in front of me the boarding pass for 1A and said, "if you want it?" The FA walked away and a moment later, the passenger decided not to move up front, and rang her F/A button to notify the FA. Door closed, we pushed, and the seat sat empty.

I wouldn't classify this as shenanigans, but if the seat was empty after the boarding was complete/door shut, they might move a friend or employee up the front. However, on the manifest the seat would have showed occupied by the revenue upgrade.