Whenever you add an entity Rhino Nature is checking the type of picked geometry. In case when regular geometry is provided (extrusion, polysurface, mesh, etc.) it will encapsulate picked geometry into a block behind the scenes. In case when a block is provided it will reuse this block. To change entity geometry or its properties you have to edit the corresponding block. In some circumstances, it may be not present in your scene and you will need first run _Insert command, place it in the scene, and then edit.