Where current service annuities and prior service annuities being paid to employees retired for service-connected disability are increased by the participating municipality pursuant to the §854.203 of the Act, such increase in the current service annuity and/or prior service annuity shall not operate to reduce the supplemental retirement benefit, if any, payable to such member, and such supplemental benefit shall continue to be paid in the same monthly amount as was being paid such member prior to such increase in the basic benefit amount.