The answer is "yes." Illinois divorce law provides that the amount of child support can be changed, up or down, after the case has been finalized. In fact, this can happen any number of times until the obligation to pay child support is eliminated.
Child support is payable to age 18 or completion of high school, but not past age 19. In order to change child support, a party must file a petition with the court and send notice to the other party. The court will then have jurisdiction to modify the child support payments in the future, starting with the date that the petition was filed. Child support payments which were due before the filing of the petition cannot be modified.
In order to increase or decrease the payments, the party filing the petition must be able to show "a substantial change in circumstances." Generally, this requires a change in the financial circumstances of one of the parties. The most common situations deal with fluctuations in income.
For example, the party receiving the child support (and let's assume that this would be the ex-wife or the mother) could lose her employment and therefore have less funds available for support of the children. She would then file a petition for an increase in support. Or more rarely, the mother could obtain a new job at a much higher salary than before. In this case, she would have more funds available for support of the children and the other party (again, let's assume that this would be the ex-husband or the father) could then file a petition seeking a decrease in support.
On the other hand, the father could lose his employment or, alternatively, find another job with a much higher salary. In the first situation, the father would file a petition to reduce support because he no longer has the income available to fund the support payments. In the second situation, the mother could file a petition to increase the child support as the father now has more funds available.
Another common scenario is where the father remarries and the new spouse enjoys a high paying job. While the new spouse's income cannot be counted for child support purposes, Illinois courts have held that her income frees up some of the father's income as it is no longer needed for expenses. Therefore, the father has more available income for child support and the mother could file her petition for an increase on these grounds.
There are even situations where the court will increase the father's child support payments even though there has been no change in his income or in mother's income. Where the children are older and their expenses have increased, some courts have awarded an increase in child support even though the income of the parties has remained the same.
It's a good idea to check with your lawyer every few years and ask him to investigate the possibility of a modification in child support. In fact, if your financial circumstances have changed or you learn that the financial circumstances of your ex-spouse have changed, then prompt follow-up with your lawyer is advisable.