see a tax pro or lawyer. there is nothing in Illinois tax law except the statute of limitations that would allow a denial of the credit on an amended return. of course, you have to submit all the required documents which would include IL-1040-x, Il-1040 as amended, form CR and a copy of the out-of-state income tax return.
Illinois enacted its income tax effective 8/1/69. as best as I can remember even back then credit was allowed for out-of-state taxes on double taxed income.
