Iowa Code § 419.6
Any bonds, issued under the provisions of this chapter and at any time outstanding, may at any time and from time to time be refunded by a municipality by the issuance of its refunding bonds in such amount as the governing body may deem necessary but not exceeding an amount sufficient to refund the principal of the bonds to be so refunded, any unpaid interest thereon and any premiums and commissions necessary to be paid in connection therewith. Any such refunding may be effected whether the bonds to be refunded shall have matured or shall thereafter mature, either by sale of the refunding bonds and the application of the proceeds thereof for the payment of the bonds to be refunded thereby, or by exchange of the refunding bonds for the bonds to be refunded thereby, but the holders of any bonds to be so refunded shall not be compelled, without their consent, to surrender their bonds for payment or exchange prior to the date on which they are payable by maturity date, option to redeem or otherwise, or if they are called for redemption, prior to the date on which they are by their terms subject to redemption by option or otherwise. All refunding bonds, issued under authority of this chapter, shall be payable solely from the revenues out of which the bonds to be refunded thereby are payable and shall be subject to the provisions contained in section 419.3 and may be secured in accordance with the provisions of section 419.4.
Iowa Code § 419.6