Had a similar situation in ATL. Wanted to SDC to an earlier flight. Got to the gate, and saw they were selling gate upgrades. Excellent, I can change and get in FC, I thought. The youngish GA, upon hearing my request, said "No, the flight is full".

Not feeling like arguing, I went to the SC, and had the change done. The SC agent didn't know why the GA said that, except it was close to shift change and she probably was going to leave very soon and didn't want to bother with it. Indeed, upon returning to the gate, a different GA was working the flight. The upgrade had already been processed. Go figure.