The membership of the Board shall, upon the determination of the Borough Council, consist of either three or five residents of the municipality appointed by resolution by the Borough Council. The terms of office of a three-member board shall be three years and shall be so fixed that the term of office of one member shall expire each year. The terms of office of a five-member board shall be five years and shall be so fixed that the term of office of one member of a five-member board shall expire each year. If a three-member board is changed to a five-member board, the members of the existing three-member board shall continue in office until their terms of office would expire under prior law. The Borough Council shall appoint two additional members to the Board, with terms scheduled to expire in accordance with the provisions of this section. The Board shall promptly notify the Borough Council of any vacancies which occur. Appointments to fill vacancies shall be only for the unexpired portion of the term. Members of the Board shall hold no other office in the municipality.