New licence to be applied for in certain circumstances. 

(a) where he desires to use the vehicle for any purpose not authorized by the licence; and

(b) where the vehicle is so altered that a higher duty or a duty of a different class is required.

(2) A new licence under this section shall not be issued until the old licence has been surrendered, and in respect of every such new licence there shall be deducted from the fee payable therefor a rebate calculated in like manner as is provided in subsection (2) of section 19.