Effective July 1, 1972, and of each succeeding year, every retirement allowance which has been in effect for one year or more shall be adjusted to that dollar amount which bears the ratio to its original dollar amount which the retirement board finds to exist between the index for the previous calendar year and the index for the calendar year prior to the date the retirement allowance became payable: PROVIDED, That the amount of increase or decrease in any one year shall not exceed three percent of the then payable retirement allowance: AND PROVIDED FURTHER, That this cost-of-living adjustment shall not reduce any pension below that amount which was payable at time of retirement.