One thing that came up during my conversation with motability is that if you have an extension to five years you must still have your car, or the delivery of your new car must be beyond your five year anniversary to get £900 GCB. So if I change before that date, say during the fourth year, I would get £700.
Needless to say unless something really exciting comes up during our fourth year, I will be making sure I see out our five year term.