Refunding warrants or bonds may be sold for cash in such manner as the governing body may direct, and the proceeds used to pay the warrants or bonds described in the initial resolution, or may be exchanged for such warrants or bonds, but no exchange shall be made at less than par plus accrued interest, and no sale shall be made at less than ninety-eight percent of par plus accrued interest on the refunding warrants or bonds. Refunding warrants or bonds may be issued from time to time as the original warrants or bonds mature or are called for payment and redemption or may be sold to pay, or, by agreement with the holders thereof, may be exchanged for, warrants or bonds which are not due. The governing body may enter into an agreement with the holders of outstanding warrants or bonds relating to an exchange of such warrants or bonds for refunding warrants or bonds and may provide, in its discretion, that the agreement shall be effective only when the holders of not less than seventy-five percent of the warrants or bonds shall have entered into the agreement.