53 Pa. Stat. § 881.317
Any municipality which has joined the system under the provisions of this Article III may, with the approval of the board, enter into a contract with the board as outlined in Article IV of this act, to increase any of the benefits enumerated in Article IV. The board shall not enter into any contract with any municipality which decreases benefits, nor shall it enter into any contract with a municipality which provides for benefits in excess of or minimum members contribution rates less than those available to it under any other existing law pertaining to the establishment of retirement systems for that class of municipality, except to the extent that excess investment earnings are allocated to provide for additional pension benefits or member accruals as otherwise provided in this law. Before the board approves any such contract it shall first determine, through its actuary, that the plan outlined in the contract is actuarially sound. Any member municipality which elects to enter into a contract for increased benefits which would result in an increase in its employes contribution rates shall first obtain the written consent of at least seventy-five per cent of its then member employes. Additional costs for contracted increases in benefits shall become the responsibility of the municipality and/or the members as specified in the contract.
53 P.S. § 881.317