Upon the entry of the final order as mentioned in section 46-583, and as soon thereafter as is practical, the board of directors of such district may determine the boundaries of each or of all the subdivisions, and make such adjustments and changes in the boundaries of such subdivisions as are necessary; Provided, if adjustments and changes of boundaries in each or all of the subdivisions are made, the directors shall be divided as nearly as possible into three equal groups, the members of the first group to hold office for the remainder of the term for which they were elected and until their successors have been elected at the first general election thereafter and shall have qualified, the members of the second group to hold office for the remainder of the term for which they were elected and until their successors have been elected at the second general state election thereafter and shall have qualified, and the members of the third group to hold office for the remainder of the term for which they were elected and until their successors have been elected at the third general state election thereafter and have been qualified. When the terms of the members of the three respective groups expire, their respective successors shall each be elected for a term of six years, and the terms of all elected successors thereafter shall be six years.