What is the test for removing a child from the custody of one parent and giving it to the other parent?

"'[A] child will not be removed from the prior custody of one parent and given to the other "unless the material facts and circumstances occurring subsequently are of a kind to render it essential or expedient for the welfare of the child that there be a change." [Citation.] The reasons for the rule are clear: "It is well established that the courts are reluctant to order a change of custody and will not do so except for imperative reasons; that it is desirable that there be an end of litigation and undesirable to change the child's established mode of living." [Citation.]'" (Speelman v. Superior Court (1983) 152 Cal.App.3d 124, 129.)