Section 67.733 - Bonds, refunding1. The revenue bonds issued pursuant to the provisions of sections 67.730 to 67.739 may be refunded, in whole or in part, in any of the following circumstances: (1) When any such bonds have by their terms become due and payable and there are not sufficient funds in the interest and sinking fund provided for their payment to pay such bonds and the interest thereon;(2) When any such bonds are by their terms callable for payment and redemption in advance of their date of maturity and are duly called for payment and redemption;(3) When any such bonds are voluntarily surrendered by the holder or holders thereof for exchange for refunding bonds.2. For the purpose of refunding any bonds issued under the provisions of sections 67.730 to 67.739, including refunding bonds, the governing body of the county may make and issue refunding bonds in the amount necessary to pay off and redeem the bond to be refunded together with unpaid and past due interest thereon and any premium which may be due under the terms of the bonds, together also with the cost of issuing the refunding bonds, and may sell the same in like manner as is herein provided for the sale of revenue bonds, and with the proceeds thereof pay off, redeem and cancel the old bonds and coupons that have matured, or the bonds that have been called for payment and redemption, together with the past due interest and the premium, if any, due thereon, or the bonds may be issued and delivered in exchange for a like par value amount of bonds to refund which the refunding bonds were issued. No refunding bonds issued pursuant to the provisions of sections 67.730 to 67.739 shall be payable in more than twenty years from the date thereof.3. The refunding bonds shall be payable from proceeds of the sales tax imposed for payment of the bonds refunded thereby. Bonds of two or more issues may be refunded by a single issue of refunding bonds.