238.31  Provision for modification of plan E.--Notwithstanding any provision contained herein to the contrary the provisions relating to retirement under s. 238.07(2)(e) shall be subject to amendment or modification by subsequent legislation and all other provisions of this chapter relating to the administration of plan E, or to the duties, rights, privileges, requirements and benefits of the members of plan E shall be subject to amendment, modification, deletion, or substitution by act of the 1965 Legislature of this state and all such legislation shall be applicable retroactively to July 1, 1963, with respect to all those persons who become members of plan E on or after July 1, 1963; provided, however, that such legislation shall not provide for a normal retirement age of members to exceed the age of 65 years, nor shall such legislation be applicable to any benefits which become payable to, or with respect to, such members prior to July 1, 1965.