Look at the help bar after you try to change to subds. It usually tells you what to do. You probably need to do
Polygons ->Cleanup and select the nonmanifold geometry option. Also, make sure that the geometry is kept to 4-sided polys, as subds can really mess up odd-sided polys.
"The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man."
George Bernard Shaw - Man and Superman