Section 1. CREATION OF SYSTEM. There is hereby created in this State a Police Officers' Pension System in all cities having a population of not less than fifty thousand (50,000) inhabitants, nor more than four hundred thousand (400,000) inhabitants, according to the last preceding or any future Federal Census; provided, however, that once such pension system becomes operative in any city, any right or privilege accruing to any member thereunder shall be a vested right and the same shall not be denied or abridged thereafter through any change in population of any such city taking such city out of the population bracket as herein prescribed, and said pension system shall continue to operate and function regardless of whether or not any future population exceeds or falls below said population bracket.