Either parent may file a petition to modify a child custody order—but usually only when there is a substantial change in circumstances that affects the child (including changes in one or both of the parents’ lives) that justifies the court revisiting and possibly modifying the earlier child custody order.

The petition to modify child custody generally must be filed in the same court in which the parents’ divorce case was decided. But if the child lives in a different county or state, the courts of the county or state where the child lives may be the proper place to file a petition to modify a child custody order.

In California, either parent has the right to petition the court to modify a child custody order. This can be done when there is a significant change in circumstances that impacts the welfare of the child, such as changes in the living situation, health, or behavior of the child or either parent. The court's primary concern is the best interest of the child, and any modification to custody will reflect that principle. Typically, the petition to modify custody should be filed in the court where the original divorce and custody orders were issued. However, if the child has since moved and now resides in a different county or state, the petition may need to be filed in the court that has jurisdiction over the child's new residence. It's important to note that the Uniform Child Custody Jurisdiction and Enforcement Act (UCCJEA) may apply if the child has moved to a different state, which helps determine the proper jurisdiction for custody matters.