I've had this happen many times and it's one of the reasons why I try to avoid AC if I can. I will normally buy a preferred seat at booking, and it seems that even a simple aircraft switch like an E90 to an E70 will knock me out of my seat. Then, the online seat selection tool won't recognize that I've already paid for the seat, the call centre says they can't do anything about it except charge me again, and writing in takes 4-6 weeks to get refunded for the double charge. It's completely stupid!