The retirement allowance, due under the provisions of sections 87.120 to 87.370, of any member who retired from service prior to February 15, 1973, with a retirement service allowance, ordinary disability allowance or accidental disability allowance may be increased at the rate of three percent per year or at the discretion of the board of trustees a cash payment may be made in lieu of such percentage increase in an amount to be determined by the board but not to exceed fifty dollars whenever it has been determined by the board of trustees that the consumer price index as published by the United States Department of Labor shows an increase of at least three percent for three consecutive months in the twelve-month period preceding, but not to exceed a total increase of twenty-five percent. Such increase in retirement allowance shall commence in the payment for the first month following the determination by the board of trustees. At least once each quarter the board of trustees shall, at a regular meeting or at a special meeting called for that purpose, consider the question of whether there has been an increase in such consumer price index.