Which State has Jurisdiction

We are modifying a child support agreement. The original divorce and child suport was calc in NC, both parents have remarried and both parents, their spouse and the children all now live in SC. Should we be filing the new agreement in SC and if so, what do we file in NC to move the jurisdiction from NC to SC?

The proper forum is SC since all parties have moved there. You will need to contact the family court in SC to have the old order registered there, and then will have to follow the SC procedure to modify support.