Surely this needs to no more complicated than every now and then the car popping up an alert on the dash saying "I've still got these devices with valid access tokens, do you want to keep them? Ok so that gives a window where it might still work but if it's once a month that's not too bad.
Or it could do it when you pair a new Bluetooth device "got a new phone? Do you want to deal register the old one?"
Mercedes do an add on "my mercedes" that you can buy and retro fit to my car, it's a plug in module so when you sell the car you can remove it, or the new buyer can unplug it and get a new one.