In what circumstances will the court change custody of children from the father to the mother?

In Forslund v. Forslund, supra, the court ordered the custody of children changed from the father to the mother because the father was 'subjecting the children 'to a rather irregular and poorly controlled living condition.'' (225 Cal.App.2d at p. 500, 37 Cal.Rptr. at p. 504.) This court found an abuse of discretion because there was no substantial evidence to show that the father's shortcomings--inability to offer a stable environment for the children, frequent moving disrupting their schooling, difficulties with his creditors--were any different than at the time of original order was made, or to show that such shortcomings affected his children's welfare. (Id., pp. 496--502, 37 Cal.Rptr. 489.)