The existing retirement and pension provisions of the City charter of said City shall remain in force and effect until the powers granted by this article are exercised by the City Council by the due enactment of an ordinance for that purpose, and such existing retirement and pension provisions of said charter shall, upon the due enactment of such ordinance, be void and of no force and effect, except as to persons theretofore duly retired and pensioned under said provisions, and as to such persons, said existing charter provisions shall remain in force.