Thank you for all the responses, everyone. The aircraft is still showing as a 787-9 but of course there's always the possibility it switched to a 330 or 340 and then back to a 789 and that messed up the seats. Someone here also mentioned there could be a fault with that seat. If that were the issue, it would be nice to be contacted about that and to be notified of the reason for the move. I suppose that's possible too, I hadn't thought of that. My plan is to show up early to the airport and see what they can do.

In the grand scheme of things, I am still in Upper Class so I can't complain too much.