I flew SAN-CLT-PHL-BDA back in October and didn't make an upgrade on the CLT-PHL leg. I asked at the lounge if there was space on the earlier CLT-PHL (first flight of the day, that I wouldn't have made if my SAN-CLT had not been early) and was told there were only 30 people on it and first was wide open. The flight was due to leave in 20 minutes but I made it with ease. I then had more time to have a snooze in PHL before getting my booked flight to BDA. So anything's possible.