This happened to me in September going AUS-JFK. Cleared into F when booking, had checked in and then a few hours later there was an equipment change and i was bumped to Y (no C+ available). They wouldnt reissue the ticket because they were afraid I would lose my aisle seat. I got the $200 voucher, which is the most they offer and did a SDC to another flight where they were able to reissue my ticket to the same seat, was #1 on both upgrade lists and ended up with C+