If you have not ever cancelled your lease early, they only charged me £50 to leave the scheme and hand back the car early. In your case I think you could order another car due to your condition change and now needing an automatic, but as you say you won’t apply for a grant again but to stay on scheme you may have to. Guess it depends on our position.
I’d be ok should I choose come off the scheme. It would depend whats cars are on the scheme, but I’m not going to be forking out silly AP’s, when I could buy one and keep it 5yrs or more and still have some capital leftat the end.