If the number is on a retention certificate the transfere fee to put it onto the next vehicle is allready paid for.
If the certificate is not in your name it will cost you £25 to change the nominee to yourself then it can be transfered to your car (providing the plate will not make the car look newer).
The easiest way to change it over is to visit your local DVLA centre, this can be done there and if you had the plates ready made up you could walk out of there, inform your insurance of the change and fit the new plates and drive off.
But they allow you 14 days to do the change once all the paperwork is completed.
If you have to fill out a form and send it all off, you'll need your V5 (log book), MOT certificate and the details from your tax disc, send this off (with any fees if nominee change) and you'll get back (eventually) the V5 with new number on it, a new MOT certificate with the new number on it and a tax disc to cover what your current tax disc covers and an envolope to send back your original tax disc.
HTH.