We had a very pleasant surprise recently regarding upgrades. We were scheduled to fly UA1957 booked in F, but thanks to a delayed arrival from SEA, we missed that flight and was booked on UA1749 in coach, since F on that flight was full. Much to my surprise, when we arrived at the gate I discovered that both of us were on the UG list. The CSR said she wasn’t sure if we would clear, and to board in our coach seats. Before the door closed, she brought us our new F boarding passes.