Tenn. Code § 17-533
An amendment or repeal of any provision of this chapter shall be made only following an actuarial determination of its effect on the soundness of the system's design as well as its effect on the cost of the system, and disclosure of such information to all interested persons. No amendment or repeal of a provision of this chapter shall affect in any way the benefits then being paid to members, retired members, or beneficiaries or benefits based on service completed prior to the date of such amendment or repeal, except as provided in § 17-534. Provided, however, that in determining such accrued benefits, average base earnings and average earnings shall be determined as of the date of the amendment or repeal, as the case may be.
T.C.A. § 17-533