3 weeks ago a similar thing happened to me. There are apparently 2 versions of the A321. From booking all the way thru my check in the day before, I had 13C, which is my favorite exit row seat. I board the plane and discover that 13 is not an exit row, but the plane is still an A321. I could not move to an exit row because they were full of pax that had non-exit row seats before the plane swap. Also, unlike United, you do not have access to the seat map on the app after you check in.