Section 7. Payment of additional tax.

Where any motor vehicle, in respect of which a tax for any period has been paid, is altered during such period, or proposed to be used during such period in such manner, as to cause the vehicle to become a vehicle in respect of which a higher rate of tax is payable, the registered owner or person who is in possession or control of such vehicle shall pay for the unexpired portion of such period since the vehicle is altered or proposed to be used, an additional tax of a sum equal to the difference between the amount of tax payable for such unexpired portion at the higher rate and the rate at which tax was paid before the alteration or use of the vehicle for that portion 1* * * * * *