If bonds issued under this chapter are not sold as in this chapter provided, the owners of unpaid warrants drawn on the county treasurer by such district for an indebtedness existing at the date of the election may exchange said warrants at the face value thereof and accrued interest thereon for bonds issued under this chapter, at not less than par value and accrued interest of such bonds at the time of the exchange; such exchange to be made under such regulations as may be provided by the board of directors of such district.