40 ILCS 5/18-112.3

(a) Persons otherwise required or eligible to participate in this System who elect to continue participation in the General Assembly System under Section 2-117.1 may not participate in this System for the duration of such continued participation under Section 2-117.1.

(b) Upon terminating such continued participation, a person may transfer credits and creditable service accumulated under Section 2-117.1 to this System, upon payment to this System of the amount by which (1) the employer and employee contributions that would have been required if he had participated in this System during the period for which credit under Section 2-117.1 is being transferred, plus regular interest thereon at the prescribed rate from the date of such participation to the date of payment, exceeds (2) the amounts actually transferred under that Section to this System.