DL has been very clear that if you board the flight, you lose your right to the upgrade. The best way to handle the situation is to not board if there are any seats available in F (since most elites board even if not UG'd but on the list, your position on the list goes up very quickly). I ask the GA if F has checked in full, then ask if F has also boarded full. If she says yes and I board, and see an empty F seat, I ask the FA if anyone is in the restroom. If so, fine. If not, I would go back to the GA to question him/her about the empty seat. It has to be taken care of before the boarding door closes.
I am confused. You say that if you board, you lose your right to the UG. Then you say if you find an empty seat after boarding, then you deplane to speak with the GA. If you boarded and you weren't #1 on the UG list, it wouldn't go to you anyway since you already boarded, no?