The transition from a five-member to a seven-member school board shall be made at the regular election following, or being held within 90 days preceding, the completion of the second regular school term during which the district maintains an average daily membership exceeding 5,000 or at the regular election following the effective date of an ordinance increasing board membership as provided in AS 14.12.030(a). Once the district has a seven-member school board, the number of members may not be changed.