I was flying on a flexible F ticket JFK-LAX and our 767 had a mechanical. They sub in a 757. At the gate I ask if I can refund my F ticket and buy a walk-up J ticket since there was a fare difference and she says no. Well how about buy a J ticket and refund my F ticket later? What about get a Y ticket and apply a VIP or my EXP upgrades. "There aren't any seats available." I get on the aircraft and there are THREE EMPTY SEATS IN FIRST In situations like these can I get a refund on the difference between F and J? What does the fare premium on this route get me???