Appointments and promotions in the Civil Service of the City shall be made according to merit and fitness, to be ascertained, as far as practicable, by competitive examinations. Ordinances may be passed to enforce this provision, to fix the powers and duties of the Commission, and to prescribe rules and regulations governing the Civil Service and determining the method by which persons in the Civil Service may be promoted, transferred, reduced or discharged; provided that all persons holding positions in the Civil Service of the City at the time of the adoption of this Charter, shall retain their positions until promoted, transferred, reduced, or discharged according to law or in accordance with such ordinances.