7. Where any motor vehicle in respect of which the tax has been paid is altered in such a manner as to cause the vehicle to become a vehicle in respect of which a higher rate of tax is payable, the person who keeps such vehicle for use shall be liable to pay an additional tax of a sum which is equal to the difference between the tax already paid in respect of such vehicle and the tax which is payable in respect of such vehicle after its being so altered, and the registering authority shall not grant a fresh certificate of registration or renew any certificate of registration in respect of such vehicle as so altered until such amount of tax has been paid.