On May 3, Illinois Governor J. B. Pritzker signed SB 2412, which eliminates the ability of qualified parties to nominate someone after the primary is over. The bill takes effect immediately.
The original bill had nothing to do with election law, but all of the contents of the original bill were erased on May 1. It passed the legislature in its new form on May 2 and was signed May 3. The bill appears to have been designed to block one particular Republican from being nominated in a legislative election this year.
There are many precedents that states cannot make ballot access more difficult in the middle of the qualification season, including one that was summarily affirmed by the U.S. Supreme Court, Hudler v Austin, in 1976.