DVLA will only change a taxation class when actually taxing a vehicle (as printed/stated on your V5).
No need to post it to the DVLA, just go to your local post office with you MOT and insurance, then point out the date of manufacture and tell them you want to change the taxation class to "historic tax".
They should tax it over the counter and give you a receipt for "nil" payment. They will also keep your V5 and send it to DVLA, who will return it to you with the new "historic" taxation class up-dated.
But, be awear some post offices are ill informed so you might have to go to a main post office.