40 ILCS 5/8-228
Whenever an employee is transferred or assigned from one position in the service of the employer to another position in such service, he shall not be involuntarily removed from membership in this fund, but may continue to participate as a contributor until because of such other position he becomes a participant in another public pension fund for the benefit of employees of the employer.
Any employee who withdraws from a position in the service which he holds because of employment in the classified civil service or career service other than by temporary or provisional appointment, and who then accepts a position in the service which would normally require his election for participation and membership, shall be continued as a participant and member in the fund while employed by virtue of a temporary or provisional appointment, unless he elects to sever his membership by making application for refund within a period of 90 days from the date of such employment in such other position, and is otherwise eligible for refund.
40 ILCS 5/8-228