Whenever refunding bonds are purchased and cancelled as provided in Section 3A-18, the taxes thereafter to be extended for payment of the principal of and the interest on the remainder of the issue shall be reduced in an amount equal to the principal of and the interest that would have thereafter accrued upon the refunding bonds so cancelled. A resolution shall be adopted by the board of the district finding these facts. A certified copy of this resolution shall be filed with the county clerk specified in Section 3A-15, whereupon he shall reduce and extend such tax levies in accordance therewith.