One reason for a change of seat would be change of guage. For instance, if at the time of booking you were on a 777-300ER and the flight is now operating by an A340-500, the seat map is going to be different. Sometimes 2 class aircraft are used as well which means that there are different seat numbers available than on a three class plane of the same type or vice versa. In general, if you book well in advance it is best to periodically check your flight to ensure that the aircraft type and cabin layout is the same as when you booked it.