Changing child custody is a special case in the law. Most lawsuits end when the parties settle or the court decides the case. If you are disappointed you can’t return to court and sue the same person over the same matter. Courts and lawyers would say your second attempt is barred by the doctrine of res judicata – a thing decided.
If you and the other parent agree or the court orders a certain custody arrangement and things change – you can sue or be sued by the same person for the same matter. The law recognizes that the thing, your wonderful child or children, can grow and change.
The key is a material change in circumstances of the child or one or both parents. If there has been such a change the court can look at the new circumstances and decide what is best for the child now. If there has not – the suit is one for the same thing that the court already decided and it is barred – res judicata.
How do you know whether a change in circumstances is material? Sometimes it is obvious but in close cases you won’t know until the judge decides.