Modification of Custody Determination
Once a custody determination has been made, a court of another state does not have authority to modify the determination, unless the state with jurisdiction determines that it does not have jurisdiction as noted above, or any state court determines that the child, parents, and any acting parents do not reside in the state which currently has jurisdiction.
To continue with the example of an initial custody determination above, let us say that Chris' father gets custody of Chris in the Iowa courts, and the mom moves to Arkansas. If Chris spends the summer with his mom in Arkansas, his mom cannot go to the Arkansas courts and attempt to modify custody - Iowa has continuing jurisdiction.
Read more about this topic: Uniform Child Custody Jurisdiction And Enforcement Act