215 ILCS 5/531.01a
Any liabilities of the Association for any member company which was an insolvent insurer as defined by this Article prior to January 1, 1986 shall be determined under the law which was in effect at the time the member company became an insolvent insurer as if there had been no amendment to that law. Any liabilities of the Association for a member company which became an insolvent insurer on or after January 1, 1986, shall be determined under the law in effect at the time when the member became an insolvent insurer, notwithstanding any prior law.
On or after January 1, 1986, any assessments made against other member companies to meet Association liabilities shall be made based on the law which was in effect when the member company was an impaired or insolvent insurer as defined by this Article. If different assessment methods are used in any one year, those assessments shall be aggregated for purposes of calculating the aggregate assessment under Sections 531.09 and 531.13.
215 ILCS 5/531.01a