§ 80. Income debentures. With the approval of the commissioner the certificate, or an amended certificate, may authorize the issuance of income debenture certificates bearing no greater interest than six per centum per annum. In the case of a housing company organized as a corporation the directors thereof may, with the consent of two-thirds of the holders of the preferred shares issued and outstanding, offer to the shareholders of the housing company, the privilege of exchanging their shares in such quantities and at such times as may be approved by the commissioner for such income debenture certificates. In no event shall the amount of shares of the housing company be less than ten per centum of the total of shares and income debentures.