The general rule is: your report all your income on your home state return, even the income earned out of state. You file a non-resident state return for the state you worked in and pay tax to that state. Your home state will give you a credit, or partial credit, for what you paid the non-resident state.

The fact that your "move" is temporaray and you intend to return to CA, means the general rule probably applies.  Illinois's residency rule appears to allow this. "You are an Illinois resident if you were domiciled in Illinois for the entire year. Your domicile is the place where you reside and the place where you intend to return after temporary absences."  https://www2.illinois.gov/rev/questionsandanswers/Pages/369.aspx

On the other hand, "made sense to move to a lower cost of living area"  seems to indicate that you no longer have a home in CA to return to. So, it could be  argued that you are actually a part year resident of both states.  So, it's not all that clear which  way you should file.  If you are maintaining your  CA drivers license, car registration and voter registration, I would follow the general rule.