I'm on UA144 SFO-ORD (777) in a few weeks and noticed that it was C0 D0 when I booked it (obviously the upgrade didn't clear!). Thinking this was very odd, I checked multiple days in either direction, and to my surprise they were all C0 D0.

I called the 1K desk and the very friendly agent said that she said there were "seats available" but the computer wouldn't let her sell them - not even as a revenue ticket. She also said that she had seen this happen a couple of times in the past week. In one case, she said she had a customer who wanted to buy a revenue C ticket but it wouldn't let her sell it. So, get this, she had to call yield mgmt to get it opened up for him (on a FULL-FARE BUSINESS TICKET!!!).

She had no explanation, and even asked around to a few of the agents next to her, but none of them knew - BTW, she agreed with me that it was extremely odd.

My only guess is that they are planning to downgrade equipment and don't want to end up with a bunch of pax in the premium cabins of a 777 when it gets switched to an A320 (going from 63 premium seats to 12 "premium" seats).