Until 6 months after the effective date, an employee may transfer to this Fund up to 6 years of creditable service accumulated under Article 3 of this Code, upon payment to this Fund of (1) the amount by which the employee and employer contributions that would have been required if the employee had participated in this Fund during the period for which credit is being transferred, plus interest, exceeds the amount actually transferred from the Article 3 fund to this Fund, plus (2) interest on the amount determined under item (1) at the rate of 6% per year, compounded annually, from the date of the transfer to the date of payment.