Thanks for the answers. From what I understand, the flights are pretty full, but I'm not looking forward to sitting in the center aisle, in a middle seat. It seems a shame that I flew so many miles on Delta and can't get an upgrade when it would be most appreciated, but I guess that's just life.

When I get to the airport in ATL, I will do my best to upgrade, but who knows? Even though my flight is showing my Delta PM status on Delta's website, it won't let me change it on the CO site. It's still showing my Continental OnePass number. I've tried to change it, but it won't let me. Maybe when I get to the airport to get my tickets (won't let me get my tickets online), I'll be able to change that. Even if I could, I wonder what will happen on my return flight, where I already have upgraded to first on the CO flight from SEA to CLE. I would hate to lose that First Class seat, which I think I received because of the price I paid for the fare in the first place. Incidentally, the fare class listed on CO's website for my flight to SEA is "Y". Will that make a difference?