<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Interesting ... I wonder if they would not allow "C" or "D" class either? Technically they are not paid "J" class.</font>
Unless you are travelling with a paper ticket, it will be hard for them to tell what actual fare basis you are booked in since all AC BP shows either J or Y. Even if the flight is handled by UA, it doesn't show the actual fare class like CP used to do.