Section 18

When any motor vehicle in respect of which a motor vehicle licence has been granted is altered, after such licence has been granted, in such a manner as to cause the motor vehicle to become a motor vehicle in respect of which a licence at a higher fee or a licence of a different class is required, the licence shall before the altered vehicle is used become void, but the holder of the licence, on surrendering the same and furnishing the prescribed particulars, shall be entitled to receive a new licence in respect of the motor vehicle, to have effect for the period for which the surrendered licence would, if it had not been surrendered, have remained in force on payment of such amount, if any, as represents the difference between the amount payable on the new licence and the amount of refund, if any, payable under section 19 on the surrendered licence.