Section 81N. Certificates of registration shall expire June thirtieth in every even-numbered year, and may be renewed as hereinafter provided. The secretary of the board shall, at least one month in advance of the expiration date of such certificate, notify the registrant of such expiration date and of the fee required for the renewal of the certificate for the succeeding two-year period, and any such certificate may be renewed on or prior to the expiration date or at any time prior to August first following such expiration date upon the payment of a fee to be determined annually by the commissioner of administration under the provision of section three B of chapter seven. If a registrant fails to renew his certificate prior to said August first, such certificate may thereafter be renewed within two years of its expiration date upon the payment of a fee as determined under the aforementioned provision. If a registrant fails to renew his certificate within two years of the expiration date, he shall be temporarily dropped from the roster of registered professional engineers or of registered professional land surveyors, subject to reinstatement as hereinafter provided. Written notice shall be given to the registrant by the secretary of the board one month in advance of such dropping from said roster. A registrant, who has been temporarily dropped from said roster, may be reinstated upon the payment of a fee as determined under the aforementioned provision and upon the filing of an application for reinstatement together with evidence satisfactory to the board that nothing has occurred during the time he was dropped from said roster which would justify the revocation of his certificate under the provisions of section eighty-one P. A registrant so reinstated shall, without the payment of an additional fee, be issued a certificate of registration.