Every retirement or death allowance payable to or on account of any member under Section 5.585 shall be increased or decreased as of July 1, 1990, and thereafter on the effective date of any legislation fixing the rates of compensation for firefighters under section 8.405 of this Charter by an amount equal to 50 percent of any increase or decrease, respectively, in the rate of remuneration attached to the rank and position upon which such retirement or death allowance was based; provided, however, that no allowance shall be reduced below the amount being received by a member or his beneficiary on June 30, 1976, or on the date such member or beneficiary began to receive the allowance, whichever is later.