I had this happen this fall, when flying DFW-ATL-XXX the day after a major sporting event being held in DFW. I purchased Y, was not UGed, called, and was told that even though no Y seats had yet sold, the UG was subject to availability and there were currently no seats available for UG.

It gets worse...

Something happened, my plans changed. Luckily, I had purchased a fully refundable fare, right? Wrong! It seems there are two classes of full Y tickets, one of which is NOT refundable without penalty. I had purchased the ticket over the phone, and would have remembered if I had been told it was not refundable. Since neither the phone agent nor his supervisor had previously heard of non-refundable Y, they allowed me a full refund anyway.

So Y fares are only instantly upgradeable and fully refundable unless they're not, which seems to be totally at DL's whim.