As I understand it, this sort of change is to keep the computers happy. There would have been a late change of aircraft and some changes to the logistics configuration. Experience has shown that the easiest, if confusing, way to handle this is to create a new flight number and shift everyone over to it. Unfortunately the confusion aspect tends to result in the aircraft running a bit late (e.g. people on connections who need checked luggage to be changed) but hopefully it's just minutes rather than hours.