To purchase a firearm or ammunition requires a current, unexpired FOID.
To transport a firearm on a public right of way, other than in disassembled condition, requires a current, unexpired FOID.
To possess a firearm in other cases requires a FOID that has been issued by the Illinois State Police. That the "current, unexpired" requirement is explicitly stated two places involving overt action, and is not stated in the passive case of mere possession, would suggest to me that someone whose FOID expired be forbidden from purchasing guns and ammo, and would be required to disassemble his firearms for transport, but would not violate the law by his mere possession.
I did a net search awhile ago for court cases on the issue and didn't find any, but given the rule of lenity it would seem an honest judge should throw out the charges on statutory grounds alone.
Surely you jest. This is Chicago. The law is what King Richard says it is. An honest judge in this town? Surely you jest. Oh, I already said that.