If the area of operation of a regional authority consists at any time of an even number of communities, the commissioners of the regional authority already appointed in the manner described in this chapter shall appoint 1 additional commissioner whose term of office shall be as provided for a commissioner of a regional authority, except that such term shall end at any earlier time that the area of operation of the regional authority shall be changed to consist of an odd number of communities. The commissioners of such authority already appointed in the manner described in this chapter shall likewise appoint each person to succeed such additional commissioner. The term of office of such person begins during the terms of office of the commissioners appointing such person. A certificate of the appointment of any such additional commissioner of such regional authority shall be filed with the other records of the regional authority and shall be conclusive evidence of the due and proper appointment of such additional commissioner.