From and after the effective date of this act, no city shall establish a policeman’s retirement fund: provided, however, that any policeman’s retirement fund, established under the provisions of sections 50-1501 through 50-1524 and which is now in effect, shall not be invalidated. Provided, further, except as in this section hereinafter otherwise provided, that no person, who shall be hereafter employed as a paid policeman in a city which has a policeman’s retirement fund at the effective date of this act, shall participate in the policeman’s retirement fund. Any person hereafter employed by a police department shall be eligible to participate in the public employee’s retirement system except as in this section hereinafter otherwise provided. Any city having an existing policeman’s retirement fund may require, by ordinance, that all of its paid policemen shall participate in its policeman’s retirement fund. No paid policeman employed in a city which has elected by ordinance to require all its paid policemen to participate in its policeman’s retirement fund shall be eligible to participate in the public employee’s retirement system.