Jesse Whiteのサイトに書いてあることが本当です。下記抜粋ですが・・。
I'm a new Illinois resident. How do I get a driver's license and automobile registration?
As a new Illinois resident, you may continue to drive on your valid out-of-state driver's license for 90 days. To obtain a new driver's license, you must visit a Driver Services facility to surrender your out-of-state license and pass a vision screening, written exam and possibly a drive exam. You must change your vehicle registration within 30 days by filling out a form at a Driver Services facility.
うちの夫も他州から引っ越してきてちゃんと筆記テストを受けていました。