If your change decreases the tax due to Illinois and you want a refund, you must file
Form IL-1040-X, Amended Individual Income Tax Return, within
- three years after the extended due date,
- three years after the date your original return was filed, or
- one year after the date your Illinois tax was paid, whichever is latest.
If your change increases the tax you owe to Illinois, you should file Form IL-1040-X and pay the tax, penalty, and interest as soon as you realize that it is owed.