Except as otherwise provided in this subtitle, the Board shall conduct an election for an exclusive representative of a bargaining unit if: a valid petition is filed in accordance with § 22–402 of this subtitle; and the bargaining unit involved in the petition is determined to be an appropriate bargaining unit under § 22–403 of this subtitle. The Board may not conduct an election for an exclusive representative of a bargaining unit if the Board has conducted an election or certified an exclusive representative for that bargaining unit within the immediately preceding 12 months. An exclusive representative or a bargaining unit in existence on June 30, 2023: shall continue without the requirement of an election and certification until a question concerning representation is raised under this title; or until the Board finds the unit not to be appropriate after challenge by the public employer, a member of the unit, or an employee organization. The appropriateness of the unit may not be challenged until the expiration of any collective bargaining agreement in effect on June 30, 2023. The Board may not modify any bargaining unit determined under existing law.