SO you have a choice.
Get a local tax professional to help you deal with it...or try to file an amended CA part-year tax return that does not include the additional IL income as a part of it's CA part-year allocation...but does include the IL income as a part of your total income for the year, then pay CA the increased taxes you "might" owe them. (a lot depends on whow much that missing IL income is, and what tax bracket you are in for CA taxes)
What I mean by that is that most state's part-year tax returns will base the %tax rate on your total income for the year , even if they only tax the income earned in that state while you were resident there. Thus if you earned $1billion in IL, and $10,000 in CA, then CA will only tax that $10,000 part-year CA income, but they will tax it at their highest progressive tax rate because your earned $1billion+10,000 total for the year. SO the additional IL income added may...or may not increase your CA tax rate....and you might owe them more for that alone. In addition, you will need to show them that it was IL and not CA income in the first place. So you do have some tax reporting work to do with the CA FTB to clear it up. Whether you can do that yourself, or thru a tax professional is up in the air at this point.
The same situation apparently doesn't apply to IL if, as you say, you've already included all that income on your IL part-year tax return........or if you didn't include that other IL income on the IL tax return, you'll have some work to do there too....but since IL uses a flat tax, the CA income level is likely to be fairly immaterial.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
