Whenever the charter of any city which adopts the provisions of this subdivision provides for a commissioner of welfare on either an elective or appointive basis, or a director of human services on an elective basis or on an appointive basis other than as provided for in this subdivision, the provisions of this subdivision shall take precedence. In such case, the incumbent in such office shall hold his office for the duration of his term provided that such term shall not exceed 2 years from the date of said adoption and provided further that if a vacancy occurs in said office, the provisions of this subdivision shall take effect immediately.