Taken from the motability site:-
If we agree to add your personalised number plates, we may also be able to update the online DVLA record on your behalf whilst you are on the phone to us or with your dealer.
We can remove your plate for you if you are returning your car or transferring number plates between cars, but we are unable to accept any responsibility for the retention or transfer of a personalised plate and you may lose your entitlement to the personalised plate if we have not been notified of your plans. Please contact us on 0300 456 4566 to tell us what you plan to do or speak to your dealer.