To initiate the process, you must file a petition with the circuit court in the county where the original order was entered. Explains how to change who makes important decisions for a child if the judge has already granted a court order.Modifying custody or support orders involves a legal process that starts with filing a petition in the court that issued the original order. Fill out the required legal forms. To begin the modification process, the party seeking the change must file a petition with the court that initially entered the divorce decree. Download and complete Divorce, Child Support, and Maintenance forms from the Illinois Office of the Courts. To change child custody or placement in Illinois, you must file a motion to modify child custody. In Illinois, postdivorce modifications to the Judgment for Dissolution of Marriage are permitted for issues of child custody and child support. To begin the modification process, the party seeking the change must file a petition with the court that initially entered the divorce decree. This relates to terms such as child support modification, modification of maintenance, or modification of your parenting time within your custody judgment.