In the event a retirement plan adopted pursuant to the authority of this chapter supersedes an existing system as hereinbefore provided, then, and in that event, the plan adopted under this chapter shall provide for the continued membership, with full service credit, of all persons who are members of the superseded system at the time such new plan becomes operative and for the continuation of all pensions, annuities, retirement allowances and death benefits to all pensioners, retired members and beneficiaries of the superseded system, in amounts not less than would have been payable to them under such superseded system.