Mich. Comp. Laws § 38.1640b

Sec. 40b. The retirement system shall recalculate each retirant's or beneficiary's retirement allowance who retired under section 26 based upon the definition of last 2 years of service added under section 3(10) by the amendatory act that added this section. If the recalculation results in an increased retirement allowance, the retirant or beneficiary is eligible to receive the recalculated retirement allowance beginning the first day of the month following the month that this section takes effect.