If I need to relocate, how do I change a court ordered custody arrangement?

You will need to have your court order modified.  In order to do so, you would need to file a motion to modify the custody order.  You and the other parent may be able to return to custody mediation.  Otherwise, there will be a court date in which both you and the other parent will testify and present evidence about the relocation.  The judge will make a decision about whether or not to modify the custody order.