If a district has terminated a plan which did provide retirement benefits and has entered into a contract to participate in the Public Employees’ Retirement System, it may adopt a plan to increase retirement benefits for former employees who had retired prior to provision being made in the contract with the Public Employees’ Retirement System for past service credit. Such plan shall be designed to provide increased retirement benefits based on cost-of-living changes as determined by the board. Payments shall be from the general fund of the district and shall be limited to the benefits set forth in the terminated plan, plus the cost-of-living increases determined by the board.