Any improvement for which bonds are voted under the provisions of this act shall be made in conformity with the report, plans, specifications, and map theretofore adopted, as above specified, unless the doing of any such work described in the report shall be prohibited by law, or be rendered contrary to the best interests of the district by some change of conditions in relation thereto, in which event the board of supervisors may, by vote of four-fifths of all the members thereof, order necessary changes made in such proposed work or improvements, and may cause new plans and specifications to be made and adopted therefor.