Mont. Code § 7-12-2191
If proceeds of the bonds or warrants of the special improvement district, including investment income, are applied to the redemption and prepayment of the bonds or warrants, as provided in 7-12-2173 and 7-12-2174, or if refunding bonds are issued pursuant to 7-12-2193 and the principal amount of the outstanding bonds of the district is decreased or increased, the assessments levied in the district and then outstanding must be reduced or increased, respectively, pro rata by the principal amount of the prepayment or the increment above or below the outstanding principal amount of bonds represented by the refunding bonds. If refunding bonds are issued, the assessments may be relevied over a term ending not later than either the final maturity date of the refunding bonds or the date 30 years after the date the bonds to be refunded were issued. The board shall reassess and relevy the assessments, with the same effect as an original levy, in reduced or increased amounts, in accordance with the provisions of 7-12-2158 through 7-12-2160.
§ 7-12-2191, MCA