To get a custody order modified is a fairly difficult thing to do.  There is a heavy burden on the party seeking the modification.  Generally the court will only change custody for the most cogent reasons.  The primary factor that the court will look at is what is in the best interest of the child.  Also the party wishing to get custody must show that they can offer superior care than the parent or guardian who currently has custody.  Besides these there are no real hard and fast rules regarding changing custody and it will be looked at from a situation-to-situation basis.