Not exactly the same issue, but similar…..
When I retired I called Motability to explain that we now only need one car (as my wife can no longer drive) and, as my private car was significantly better than anything available on Motability, could we return the Motability car early. They said yes. I also asked if we wished to rejoin at any time thereafter was that OK. Answer also yes. As it happened I sold my private car as it wasn’t an SUV and my wife finds the higher seating position easier! My sense is you can rejoin but only one way to find out for sure – give them a call.