When you say "ex," I presume that you have been through the court process and that is why you two have split custody of the kids. If you have found out information SINCE the last court order which indicates that the orders should be different, you can file a motion to alter those orders. Even if the data preceded the last court order, if you or the court did not know about this info that you believe is important to child custody, it can be raised now under the Castle v. Simmons case.
As a general matter, you can alter custody and visitation orders by filing a motion for that purpose. The flow-chart, legal tests, and a great deal of other information about how it works is set out on the Child Custody page of our firm website, at http://www.willicklawgroup.com/child_custody_visitation. Generally, unless there are doctor or police reports, etc., one "proves" unfitness or harm by way of a report from an outsourced evaluator. But the question will be raised of why you did not do so earlier.
If you decide you would like to discuss this with one of the attorneys of this firm, please take a look at our consultation policies, procedures, and costs, at http://www.willicklawgroup.com/consultation_policies.
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