I had a similiar experience last week on NW. I had laready been confirmed, via EUA in first class. I was doing a standby for an earlier flight, the ticket counter said They can only assign FC seats at the gate,
since you're a standby. I then headed for the worldclub, the said the same thing. The NW agent then said "I can try and switch you to the earlier flight. Then she gives me a puzzled look and says "seat 2a, same as you had before, looks like it worked" I said "sounds good to me".