Ill. Admin. Code tit. 80 § 302.500
Certified and probationary employees may voluntarily request or accept assignment to a vacant position in a class having a lower maximum permissible salary or rate. All requests for or acceptances of such voluntary reductions shall be in writing, and shall be authorized by the employee and be directed to the head of the agency in which the vacant position exists. A certified employee who is assigned and accepts a voluntary reduction in grade shall be certified in the lower class without serving a probationary period and a probationary employee, the balance of the probationary period; provided however, if reduction results in return to a trainee class or other class for which there is no provision for certification in said class, the individual's certification shall be terminated.
Ill. Admin. Code tit. 80, § 302.500