Try setting triangle count at offset 68 in the surface handle to zero and increment new total verts changed at offset 4 in the mesh header (at offset 644 in the mesh entity). Might do the trick.
Or, as I have just found
Set the vert count at offset 64 and the tri count at offset 68 in the surface handle to zero.
Or, as I have just found
Set the vert count at offset 64 and the tri count at offset 68 in the surface handle to zero.