On and after July 1, 1980, and to the extent that the necessary funds are appropriated therefor by the General Assembly, whenever the retirement benefits of beneficiaries under this chapter are increased pursuant to Code Section 47-3-126, the board of trustees shall make a corresponding increase in the retirement benefits of retired public school teachers who retired prior to July 1, 1978, pursuant to a county, municipal, or local board of education retirement or pension system. Such corresponding increase shall be equivalent to the percentage increase in retirement benefits of beneficiaries under this chapter. For the purposes of this Code section, such retired public school teachers shall be deemed to be members of the retirement system. The board of trustees shall pay directly to each such retired public school teacher the increased retirement benefits provided for in this Code section. Increases in retirement benefits granted on and after July 1, 1980, to such retired public school teachers by a county, municipal, or local board of education retirement or pension system shall not reduce or replace the increase in retirement benefits payable to such retired public school teachers under this Code section. If the General Assembly at any time appropriates some but not all of the funds necessary to fund the increase in retirement benefits under this Code section, then the increased retirement benefit otherwise payable under this Code section shall be reduced pro rata by the board of trustees in accordance with the funds actually appropriated by the General Assembly for such purpose.