Colo. Rev. Stat. § 32-7-126
No general obligation or revenue bonds may be refunded unless the holders thereof voluntarily surrender them for exchange or payment, or unless they either mature or are callable for prior redemption under their terms within ten years from the date of issuance of the refunding bonds. Provision shall be made for paying the bonds refunded within said period of time. No maturity of any bond refunded may be extended over fifteen years. The interest rate on such refunding bonds shall be determined by the board. The principal amount of the refunding bonds may exceed the principal amount of the refunded bonds if the aggregate principal and interest costs of the refunding bonds do not exceed such unaccrued costs of the bonds refunded, except to the extent any interest on the bonds refunded in arrears or about to become due is capitalized with the proceeds of refunding bonds. The principal amount of the refunding bonds may also be less than or the same as the principal amount of the bonds being refunded so long as provision is duly and sufficiently made for the payment of the refunded bonds.
C.R.S. § 32-7-126