The Commission may establish a merit system that includes all of its employees except: the general manager, secretary, treasurer, and chief engineer; the head of a department; the inspector general; and any temporary employee of the Office of the Inspector General appointed under § 17–603 of this article; and a part time, temporary, or contract employee. The Commission may: appoint or promote a merit system employee to the position of general manager, department head, or inspector general or designate a merit system employee as the general manager, department head, or inspector general on an acting basis; retain the employee in the merit system in the position or grade from which the employee was appointed or promoted; and continue to exclude the positions of general manager, department head, and inspector general from the merit system.