Can you Remove xClass relationship with onChange field event?

I'm trying to clear out the xClass relationship when the class of a part is changed.  The code I have below is ran on the filed event of the drop down onChange, however it doesn't remove the xClass relationship.  

var rootItem = document.thisItem;
rootItem.fetchRelationships("Part_xClass");
var xClassRelationship = rootItem.getRelationships("Part_xClass");

if(xClassRelationship.getItemCount() > 0){
    var current = xClassRelationship.GetItemByIndex(0);
    rootItem.removeRelationship(current);
}

return rootItem;