Searching: "If I amend my federal return to change from standard deduction to itemized, to match my wife’s filing, do I need to amend my state (Illinois) return also?"

The Illinois tax return does not utilize standard or itemized deductions.  The return starts with your federal adjusted gross income amount and adds back items taxable in Illinois that were not at the federal level and subtracts anything that was taxable at a federal level but is not taxable at the state level.  Therefore, changing from itemized to standard on your federal return should not affect your Illinois tax return.  If your Adjusted Gross Income amount changes on the 1040 (line 7), then you would need to amend your Illinois return.