Last week I booked UA 476 DEN-PHL and saw that it was a 319 and so took seat 12A (Lots of legroom). However, yesterday I used Easy Checkin for the flight and the web page said that it was on a 320, but the seatmap was still for a 319. Deciding not to take a chance, I changed to 11C, and sure enough, it turned out to be a 320 and 12A would have been in E-.

I realize that sometimes UA changes from a 319 to a 320, but it raises the question, how often is the seatmap on easy checkin different from the actual plane configuration? Obviously some part of United's database knew that the flight was on a 320 since it was reflected on the checkin page. But it didn't carry over to the seatmap.