Even where the circumstances of a case have significantly changed, a court will only modify a custody order if it is in the best interests of the child. (info.legalzoom.com)
The court will modify a custody order when one parent establishes that there has been a «substantial change in circumstances» since the court issued the first order. (divorcesource.com)
The court will only modify a custody order upon a demonstration of a material change of circumstances. (divorcesource.com)