The retirement allowance of any beneficiary of a superseded system not entitled to an increase in retirement allowance under §8-36-708 shall be increased as of July 1, 1972, as follows. The retirement allowance which would otherwise be payable without regard to the election of any optional modification shall be increased by the excess, if any, of an amount computed on the basis of §8-36-708(a)(2) over such retirement allowance.