Any drainage district of Missouri heretofore organized, in process of organization or that may hereafter be organized under the provisions of any previous or existing laws of this state, either general or special, may elect in the manner herein provided to become and be reorganized under the provisions of sections 242.010 to 242.690; provided, that such reorganization shall not be required of districts heretofore organized or in process of organization at the time of passage of this law under the provisions of article 1, chapter 41, of the Revised Statutes of 1909, but all such districts shall have the right to proceed under the provisions of said sections.