In Bergeron v. Bergeron, 492 So. 2d 1193 (La. 1986), the Supreme Court imposed a heavy burden on a party attempting to persuade the court to change a "considered" degree in a custody case. That party must prove that the continuation of the existing custody plan is so deleterious to the child that a modification of the custody decree is warranted. The burden in such a case is clear and convincing evidence that the harm likely to be caused by a change of environment is substantially outweighed by its advantages to the child. Bergeron v. Bergeron, supra.