The board of supervisors of any drainage district in the State of Nebraska organized under and by virtue of the provisions of sections 31-301 to 31-369, and any amendments thereto, which has issued, or may hereafter issue valid interest-bearing bonds under the provisions of said sections, and which bonds are now or may hereafter be outstanding and unpaid, may take up and pay off any such bonds whenever the same can be brought about by lawful means, by the issue and sale or the issue and exchange therefor of the refunding bonds of such drainage district; but bonds so to be issued shall not exceed the amount lawfully owing and unpaid upon the bond or bonds so sought to be taken up and paid. Bonds so issued shall not bear interest greater in rate or amount per annum than the bonds so sought to be taken up and paid, and shall be made to run for not more than twenty years from date of issuance.