The refunding bonds may be sold or exchanged in installments at different times or an entire issue or series may be sold or exchanged at one time. Any issue or series of refunding bonds may be exchanged in part or sold in part in installments at different times or at one time. The refunding bonds may be sold or exchanged at any time on, before or after the maturity of any of the outstanding notes, bonds, certificates or other obligations to be refinanced thereby.