Had a similar issue from SNA-SFO the other day. When I went to check-in on-line day before it said A320 and seat map was also A320, which was the originally scheduled equipment. I had 3C, but for some reason thought to myself, let's switch to 1D (the only seat left in rows 1 or 2), good thing I did, because it ended up being switched to an A319 and sure enough several people who were in FC row 3 were now given seats in economy.