First of all take the plate from your car using form v317 and choose to retain it.DVLA will then issue a new plate for your car and a retention document v778 for your cherished plate.Sign this and give it to your car dealer.It will cost £80 to take the old plate off and assign it to the new car. You will pay again when the new car goes back if you want to retain your plate.Dont forget to notify your insurance company.Most transactions can be done online once you have the v778.