Hopefully you will be able to amicably resolve this with your ex as well. If not and your separation agreement was incorporated into the divorce judgement, you will have to file a motion to modify custody in that same docket number. If it was not incorporated, you have to file a Complaint for child custody and custody is determined de novo (as it has never been litigated).
Is the child home schooled? How would he or she be able to live six months in one state and six in the other? Is that in the child's best interest? What is the motivation for the move? There are relocation cases (see blog link below) that require the judge to consider certain factors in deciding whether the child will live with you or your ex if the judge determines primary placement should be with one parent over the other.
Consult with an attorney!!!
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