Payment of additional tax

Where any motor vehicle in respect of which a tax has been paid is altered in such manner as to cause the vehicle to become a vehicle in respect of which a higher rate of tax is leviable, the owner of such vehicle shall pay as additional tax a sum which is equal to the difference between the tax already paid, and the tax which is payable, in respect of such vehicle for the period for which the higher rate of tax is leviable in consequence of its being altered.