The commission, by rule, may adopt a system under which licenses expire on various dates during the year. For the year in which the license expiration date is changed, license fees payable on a specified date shall be prorated on a monthly basis so that each licensee shall pay only that portion of the license fee that is allocable to the number of months during which the license is valid. On renewal of the license on the new expiration date, the total license renewal fee is payable.