Illinois' sourcing rule is irrelevant to Illinois residents, because Illinois residents are taxed by Illinois on all their income, regardless of its source.
New York non-residents are taxed by New York on New York-sourced income. By New York's convenience of the employer rule, your remotely-earned income is considered New York-sourced and thus taxable by New York..
Therefore the income you earned remotely after moving to Illinois is taxable by both states, making you eligible for an "other-state credit" per to the 2015 U.S. Supreme Court ruling.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
