Maya has safeguards to prevent you from editing the smoothed mesh directly. In most cases you should only edit the original version of the proxy mesh, not the smoothed mesh. However, in some cases you can subdivide and edit components on a few faces, or soften or harden a few edges. In general, you should wait until the model’s topology is finalized before editing the smooth mesh.
To disable the safeguards that prevent you from editing a smooth mesh
As a precaution, turn the attribute off again after you have finished editing the smooth mesh.