Sec. 18. OPERATION OF FUND NOTWITHSTANDING CENSUS CHANGE. Any city which has heretofore established a firemen and policemen fund in accordance with Article 6243B of Vernon's Texas Civil Statutes or as amended, shall continue to operate such fund under the provisions of this Act. It is further provided that the fact that any future Federal Census may result in said city being above or below the population bracket herein specified shall not affect the validity of such fund and such fund shall continue to be operated pursuant hereto.