One key to being a successful parent after divorce is to clearly and directly follow the law. It is never a good idea to sneak around or do anything that a court would consider illegal. Sneaky, underhanded actions can result in penalties for the parent, ranging from fines to losing custody. In an effort to understand how to act transparently, consider a post-divorce scenario where the court order (typically the Judgment) provides that the son will live with mom. He is, however, actually living with his father, even though father does not have court-ordered custody. In this case, the father is technically in violation of the divorce judgment; the new arrangement is, after all, not what the judge ordered. Even if Mom approves of the child living with Dad, the situation still lacks a judicial "stamp of approval."
In this scenario, Dad should go to court and request a formal modification of child custody. The judge would consider a new custody arrangement and presumably approve it if both parents were in agreement. The new custody agreement would then "of record" and available for future review if need be. If, however, Mom objected to the change in custody, the judge would have to assess the new arrangement more thoroughly. As with many cases of child custody, the judge would examine whether the new arrangement served, typically using the "best interests of the child" standard of review. The judge's conclusion would then determine whether the new custody arrangement was approved and made "official" or whether the original court-ordered custody should remain in place, or perhaps even a completely different arrangement.