I’ve been with 4 different dealerships and everyone of them have refused point blank to adjust AP stating it’s against motability policy .
It’s true that Motability don’t like dealers doing this, but the majority will do so happily as it doesn’t make any difference to what they receive for the car. It’s just a 5 minute admin job on the Motability system.
It would pay you to establish if the dealer is happy to do this before placing any order with them, and if they say that they won’t try another dealership. With our previous car (BMW) the AP dropped by £100 and as it was a small amount I didn’t call the dealership, but I didn’t need to. They called me and said that they had adjusted the AP without me even asking, saying “that £100 is better in your pocket than theirs”