All or any part of such refunding bonds may be exchanged, dollar for dollar, for the bonds to be refunded, or they may be sold, at not less than their par value, as directed by the board of commissioners and the proceeds thereof shall be applied only to the purposes for which said refunding bonds are issued. The authority vested in the board of commissioners by the judgment and decree as in this chapter provided shall be and remain effective until all of the bonded indebtedness so authorized to be refunded has been paid, redeemed or refunded.