ChangedVids/ChangedElementIDs is allowed to have new elements, since it is natural to split UVs. However, ChangedConnectivityTids must not have new Tids, because the tids form our correspondence between the unwrap mesh and the original layer. Also note that if you are gathering the changed vids/elements based off of triangles, you should gather them from the post-change mesh/overlay so that you do not miss any added elements, and since removed elements are captured by changed tri connectivity.