The department may from time to time or at any time sell or exchange refunding debentures for the purpose of retiring, paying or refunding either all or part of the outstanding debentures or obligations, or of one or more series thereof, as it deems advisable. Refunding debentures may be issued and delivered as outstanding debentures or obligations to be refunded thereby, if the debentures are mature or are about to mature or are subject to call or redemption, or if the retirement thereof has been assured by consent of the holders thereof. Refunding debentures may be delivered in whole or in part in exchange for outstanding debentures or obligations with the consent of the holders thereof.