For purposes of distributing the reregistration work of the department uniformly throughout the year as nearly as practicable, the boards in the department may, with the approval of the director, extend by not more than six months the date fixed by law for the renewal of any license, certificate or permit issued by them, except that in such event any renewal fee which may be involved shall be prorated in such manner that no person shall be required to pay a greater or lesser fee than would have been required had the change in renewal dates not occurred.