Subject to the vested rights rule, the Board of Supervisors is empowered to enact, by a vote of three- fourths of its members, ordinances to amend the provisions of the Retirement System to conform with the community property provisions of Civil Code Sections 4800 and 4800.8, provided that the Board of Supervisors shall first secure, through the retirement board, an actuarial report of the cost and effect of any such change and provided further that any such ordinances shall not increase overall Retirement System benefits costs.