While that appears to be what happened (and I've seen the reverse) the swap occurred at least two weeks prior to the flight as he said that was the equipment posted when he booked the tickets. I would have to assume if several people grabbed the offered TOD's those 4 additional seats which "didn't exist in inventory" would certainly be sold as available inventory. They were displayed on the seat chart as open fwiw for the past couple of weeks. It did showed at least J6 for at least up to 72 hrs prior to the flight with the TOD buy up price as I stated ($550'ish). I still have to say that UA indeed will try to sell all F seats down to the wire (and quite honestly why not). As you noted, once the unsold inventory was "released" at the gate then obviously several people got the upgrades including my business partner who stood at #1 on the list for days.