Once a dealer places the order on the Motability system, the AP is fixed. If it goes up you don’t pay the increase. If it goes down, the dealer is able to cancel and re-order on the Motability system without affecting the ‘proper’ order, which is between the dealer and the manufacturer.
Where this sometimes goes wrong is if the dealer hasn’t inputted the order on the Motability system before the AP changes. A few people have been caught out by this.
However, you mention a “change of model” and whilst you haven’t given any detail about this, it sounds as though you might have ordered a model that is no longer available and you are now getting the new / replacement model, which has an AP that is £200 more. Is that correct? If so, technically the dealer is correct. If the dealer isn’t willing to match the AP for the previous model then I’d be inclined to call Motability and see if they will make a goodwill gesture and match the previous models AP. I’ve not known them do it, but given the cash surpluses that they make it is clear that the APs that we pay are much higher than needed to cover the costs, so you would hope that there is some discretion in cases such as this.
Let us know how you get on (as well as more details on the model) as it will be of use to others in a similar situation.