Hello! I'm traveling to London in October, and about a month ago I noticed that I no longer have an assigned seat for my flight home out of LHR. As far as I can tell this happened because the airline changed the plane that had been assigned for my flight, as the new seating map has fewer rows than it did when I first booked. It won't let me choose new seats, saying that they are "unable to assign seats for everyone" and that I should keep checking back or wait to be assigned a seat at the airport. I've been checking back every few days for a while now, and so far no seats have opened up. I'm just wondering what I should expect at the airport if seats never open up. Will I still be allowed to checkin online without a seat assignment? Am I likely to be bumped from my flight?

I'd try to reroute my flight now, but the flights for that date that use the same number of AAdvantage miles involve much longer travel times and/or are less convenient. Plus being bumped and receiving compensation for it wouldn't be the end of the world.

Thanks in advance for any insight offered!