In all cases where any board of directors of any township library have issued or may hereafter issue bonds, or other evidence of indebtedness for money on account of any public library building, or for the improvement thereof, which remain outstanding and which are properly authorized by law, such board of directors may, upon the surrender of any such bonds or other evidence of indebtedness, or any number thereof, issue in place or in lieu thereof, or to take up the same to the holders or owners of the same, or to other persons for money with which to take up the same, new bonds or other evidences of the indebtedness in such form, of such amount, upon such time, not exceeding the term of twenty years, and drawing such rate of interest, not exceeding six per centum per annum, as may be determined upon, and such new bonds or other evidences of indebtedness so issued shall show that they are issued under this act. Provided, that the issue of such new bonds in lieu of such indebtedness must be authorized by the legal voters of such township voting at a referendum initiated as provided for in Section 1 of this act and conducted in accordance with the general election law: and, Provided, further, that such bonds or other evidence of indebtedness shall not be issued so as to increase the aggregate indebtedness of such township beyond five per centum of the value of the taxable property therein, to be ascertained by the last assessment for state and county taxes, prior to the issuing of such bonds or other evidences of indebtedness.