However, custody orders may not be permanent, and either parent has the right to request modification of a custody order if the circumstances change. (info.legalzoom.com)
Generally, you may request modification of a custody order if there has been a substantial change of circumstances that were unknown at the time the judge made the initial order. (info.legalzoom.com)
In addition, because child custody relocation can work a substantial change in circumstances, the parties may also need to seek a court modification of custody or visitation order. (divorcesource.com)