There shall be a judge of probate for each probate court. The probate judge of each county holding office on June 30, 1976, shall continue to be such judge of probate until the expiration of his term of office at which time his successor shall be selected as provided by law for a term of four years and until his successor is elected and qualifies. Except as otherwise provided by this section, any vacancy in the office of probate judge shall be filled as provided by law.