Motability do have a bit of a ‘get out’ clause about this situation though – from their website:
Please note that if the specification of the car changes after the application, the price may be subject to change.
https://www.motability.co.uk/contact/faqs/what-happens-when-prices-change
However, should the dealer no longer be able to supply you with the vehicle you ordered at the AP at the time of order, they cannot force you to pay more for the revised specification. You need to speak with Motability who may well release you from your order leaving you free to choose another vehicle on the scheme.
Not ideal if you had set your heart on a particular vehicle, but as manufacturers update vehicles with new specs, coupled with Covid related delays, it is, sadly, likely to happen on occasions. Particularly if there is a long time span between order and the vehicle being built.
What you could do as well, is see if any other dealers have your chosen vehicle ‘in stock’ at the current AP.
Alternatively see if you can get a Motability Grant to cover the diffference, particularly if it is a substantial difference.