I had this happpen on a last minute full J international ticket where i had two US segments booked into econcabin due to full front cabin and on YVR-DFW they upgraded someone from economy as the door closed. I was furious. At the DFW F lounge the agent explained the codes to me so this wont happen again. This is a terrible glitch that they really ahould fix. Paying $7k and watching someone on an econmy ticket get an open F seat over me was painful.