I had the same thing happen recently, though I'm 99% sure that the protected segment was not actually upgraded or in F. I say this, because when I looked at the upgrade list for that flight, I was not on there, and someone else was with the seat that I had been assigned.

Fortunately, I made my original flight, but I'm pretty sure that had I shown up at the gate, that I would have been given a seat in the back.