I also have this happened to me on quite a few times since I always booked months in advance. This is another one of those AC IT failure. There is usually a fairly long lapse time between the equipment change and notification time (if any) and by the time you received the notification, your selected seat is gone. The only way I have found to keep abreast of equipment change is to subscribe to the paid version of Expert Flyer to monitor the equipment change. Once a change is detected, you will receive a notification from EF and you can then re-select your seat before the automatic seat assignment.