The governing body of a public agency that has established a pension trust or retirement plan funded by individual or group life insurance or annuity contracts may, notwithstanding any provision of this part to the contrary, enter into a contract to participate in this system making its employees members of this system, and continue the trust or plan with respect to service rendered prior to the contract date. A pension trust or retirement plan so continued shall be deemed not a local retirement, pension, or annuity fund or system for the purpose of this chapter. The public agency shall have all the rights of any other contracting agency to provide prior service benefits for its employees but may elect in the contract instead not to provide a benefit with respect to prior service, in which case the service rendered by its employees prior to the contract date shall be deemed not to be state service.