I was forced against my will to leave MB in 2017 after previous attacks on the disabled by are wonderful governments. Back then MB was helping us with a £2k leaving payment because we were unaware of the change in policy of the then government. If you renewed your car after a given date, that I can’t remember what it was now, you would not get that leaving payment because you would be aware of how disrespectful towards the disabled by government had become. Not sure if they help these days if you have to leave due to no fault of your own.
My lease in 2017 just happened to end at the same date I changed from DLA to PIP or not PIP in my case so maybe the DWP planned it that way.
I never got the GCB because of the £2k leaving money and no AP money back because the lease had ended but like kezo said above I think they pay you back the AP on a pro rata basis.