(a) A person eligible for early retirement, transferred to a position which does not qualify as a person so eligible after at least ten years of service as a person so eligible may elect to continue to contribute at the additional rate of compensation, for which he shall receive full benefits after twenty years of service or he may elect to pay six percent of compensation with reimbursement or credit for overpayment, but shall not be eligible for early retirement.
(b) A person in a position which does not qualify as a person eligible for early retirement who is transferred to a position so eligible may elect to contribute at the additional rate of compensation for which he shall receive full benefits after twenty years of credited service, provided that such employee has been eligible and paying at the higher rate for at least ten years. Such employee shall be required at the time he becomes eligible for early retirement at the time he elects to become such pursuant to section 718c of this chapter, to pay retroactive to December 29, 1986 or for credited service to the time he makes such election to pay the difference between the rates for the time he seeks credit.