How do I legally change my state residency?

Go to the DMV and get an Illinois License, and start using your local address as your legal address.

Have you still been voting in California? If you have, it could delay your claim of residency.

Also register to vote in IL and if you have a vehicle, register it in IL.

If you’ve been paying Illinois income taxes then you’re halfway there. Really, all you have to do in that case is file a Change of Address card with the Post Office, register your vehicle and obtain an Illinois driver license.

If you’ve just been a student in Illinois, and not employed there (other than part time work during school), then you haven’t established residency there yet. Still, whether you register to vote or not, registering a vehicle is one of the best ways to say “I’m a resident”.

Registering to vote cements the whole thing.

If you want residency for school purposes, most schools set their own residency requirements. Sometimes it’s as simple as living somewhere for a set amount of time; other places you need to be registered to vote, have a driver’s license/ID, have proof that you’ve been living there, etc.

Some places will require a utility bill with your name on it to be able to get a new driver’s license (Tucson required that), so call the DMV before you spend 5 mins to 3 hours in line (depending on where you go) to find out what they need to prove your residency. There’s nothing worse than sitting for 3 hours to find at the desk that you don’t have everything you need (again, a Tucson experience.) Some other places only require your verbal confirmation that you live there.

I don’t think there is one single official way to establish residency in a state. I wanted to apply to a graduate program at U.T. Austin and moved down there about 14 months prior in order to establish residency so I could pay instate tuition and the University required that I be resident of the state for a year in order to do that. But the state only required, I think that I be a resident for 30 days before I could register to vote. The state also had laws that required you, upon moving to the state to register your vehicle and get a Texas driver’s license within a certain period and I think that was also 30 days. You usually, for the 30 day thing have to show a utility bill with your name on it that shows you have lived in the state for that period as someone mentioned. So the University, the DMV and the the state, for voting purposes kind of had different definitions of what it meant to be a “resident.” When I got there, I just did anything and everything I could as soon a possible to ensure that I would be considered a resident for tuition purposes. Registered my truck, got a TX drivers license, registered to vote (would have done that anyway), got a library card, made sure my name was on the lease on our rental house.