Child custody is not set in stone. Courts sometimes modify custody when either of the parents request it with good cause. Modification leaves intact the general purpose of the order but amends its details. After a divorce, courts entertain motions to modify child custody (and support) because of a change in circumstance in the lives of the either the custodial or the noncustodial parent. Changed circumstances very often involve amending a custody and visitation routine that has broken down.

For example, courts entertain motions for modification when the custodial parent interferes with the rights of the noncustodial parent. In one Missouri case, the court said that the facts of a case showed "an attempt by one parent to alienate a child from the other parent is a changed condition and can form the basis for modification." Interference need not be an active campaign to alienate a child from the noncustodial parent; interference with "reasonable" visitation can be grounds for modification.

One variation on interference with the reasonable visitation by the noncustodial parent sometimes happens when the custodial parent removes a child from the jurisdiction where the family resided before the divorce.

Not infrequently, shared custody routines break down when parents demonstrate an inability to cooperate, extreme anger or hostility, serious misconduct, make false allegations about one another, or neglect the child.

In such modification, courts work under the assumption that it is generally in the best interest of the child to have a continuing relationship with both the custodial and the noncustodial parent, and behavior that erodes that is sometimes grounds for modification.

Sometimes shared custodians request the court to modify the physical custody arrangements while leaving shared legal custody in place. This may happen as a result of interference by the primary physical custodian, or his or her instability, or his or her remarriage.