Sec. 7. MODIFICATION OF BENEFITS AND ELIGIBILITY. (a) The board of trustees of a retirement system may change the benefits or eligibility requirements for benefits payable from the retirement system, may provide for reinstatement by a member of service credit previously forfeited, and may adopt or change other requirements for the payment of benefits, except as otherwise prohibited by this Act.

(b) Before a board of trustees chooses to adopt or change a benefit or requirement for payment of benefits under this section, the proposed addition or change must be approved by:

(1) an eligible actuary selected by the board; and

(2) a majority of the participating members of the retirement system voting on the addition or change by secret ballot at an election held for that purpose at which at least 50 percent of all participating members of the retirement system vote.

(c) To be eligible to approve an addition or change under this section, an actuary must be either a fellow of the Society of Actuaries or a member of the American Academy of Actuaries.

(d) Except as provided by Subsection (e) of this section, if a board chooses to adopt an addition or change after it has been approved as provided by this section, the addition or change applies to all persons who are participating members of the retirement system on the effective date of the addition or change and all persons who became participating members during the time the addition or change remains in effect. The addition or change also may apply to:

(1) persons receiving monthly benefits; or

(2) former members of the fire department who meet an applicable length-of-service requirement for service retirement.

(e) An addition or change adopted under this section may not, without the written consent of the member, retiree, or eligible survivor under Section 15 of this Act, deprive a member of the retirement system, a retiree, or an eligible survivor of a right to receive a vested accrued benefit.

(f) The effective date of a change or addition adopted under this section is a date specified by the board of trustees that is not earlier than the date of adoption by the board. A change or addition may not be applied retroactive to its effective date unless required to maintain a plan's tax qualification status.