Whenever a vacancy for an officer shall have occurred in headquarters, South Carolina Air National Guard, except that of chief of staff, it shall be filled from among the officers of the South Carolina Air National Guard who are considered best qualified to fill the vacancy by the chief of staff, headquarters South Carolina Air National Guard, and the Adjutant General, provided that no officer shall be assigned to headquarters, South Carolina Air National Guard, until he has completed a total of three years commissioned service in the South Carolina Air National Guard. Assignment of an officer to headquarters, South Carolina Air National Guard, does not entitle him to promotion without regard to seniority rights specified elsewhere in this chapter. Position vacancies for enlisted personnel in headquarters South Carolina Air National Guard shall be filled as directed by the chief of staff, headquarters South Carolina Air National Guard, subject to approval of the Adjutant General.