In Baldwin County, mayors of municipalities that participate in the Employees' Retirement System of Alabama may participate in the Employees' Retirement System of Alabama upon the same terms and conditions as may be specified by law for any other employee in the same retirement system. A mayor holding office at the time of the ratification of this amendment shall be eligible to purchase service credit in the Employees' Retirement System for the time the official has served in the current office. No person may participate in both a supernumerary program and the Employees' Retirement System based on the same service.