Child custody rulings often seem "set in stone," especially if you are the parent who did not receive primary custody. But it is important to remember that child custody agreements can be changed over time, either through a mutual agreement by the parents or by filing a motion to modify.
In both Missouri and Illinois, courts will generally only grant one parent's motion to modify custody if the parent can demonstrate that there has been a substantial change in circumstances. This can include changes in working hours, a parental relocation, or certain changes in behavior or circumstances that pose a risk of harm to the children. Sometimes, the non-custodial parent will file a motion to modify custody due to fears that the custodial parent's new boyfriend or girlfriend is a danger to or a corrupting influence on the children.