In what circumstances can child support be modified, according to Illinois law? One situation in which Illinois law deems a child support modification appropriate is if there are significant changes such as an unexpected injury or a child’s needs increase. An additional situation is if a parent’s income increases either by 20 percent, or by $50 per month. If you are seeking a child support modification, contact Law Office of Bradley R. Tengler, P.C. We will advocate for the best interests of you and your family.