Colo. Rev. Stat. § 37-43-147
In case of the sale of said refunding bonds or any portion thereof for cash, the same procedure shall be followed as in case of the sale of an original issue of bonds; but, if no bid of par or better is received at public sale, said bonds may be sold at private sale. No bond shall be sold for less than ninety-five percent of the par value thereof. The money realized from said sale shall be used only for the purpose of paying the indebtedness for which said refunding bonds were issued. Any part or all of said issue of refunding bonds may be exchanged for the indebtedness to be refunded; but the par value of the refunding bonds exchanged shall not exceed the par value of the indebtedness refunded thereby. All indebtedness so discharged, either by payment or exchange, shall immediately be canceled by the county treasurer of the county in which the office of the district is located.
C.R.S. § 37-43-147