The members of any annuity fund, benevolent association, or retirement system of any political subdivision, hereafter referred to as a local pension system, may elect to become members of the retirement system by submitting to the Board a petition duly signed by a majority of the members. The Board may approve the participation of such members in the retirement system as though the local pension system was not in operation, and the provisions of this article shall thereupon apply. Existing pensioners or annuitants of the local pension system who are being paid pensions on the effective date of coverage and former employees whose rights are vested may retain their rights under the local pension system and be paid at their existing rates by the retirement system. After the local pension system is discontinued, the rates may be increased by all future percentage increases which are granted to beneficiaries retired under the provisions of this chapter. If deemed practicable by the Board, any cash and securities credited to the local pension system shall be transferred to the retirement system as of the effective date of coverage. The administrative head of the local pension system shall certify the proportion of the funds of the system that represents the accumulated contributions and the relative shares of the members. If appropriate, the accumulated contributions and shares shall be credited to the individual accounts of the members in the members’ contribution account. The operation of the local pension system shall be discontinued as of the effective date of coverage. If the Board determines that the transfer of funds is not practicable, the Board may enter into an agreement with the employer to coordinate any benefits payable under this chapter with any vested benefits payable under the discontinued local system.