By popular demand here is a tool that lets you convert skin to physique and back to skin using the vertex number. If you edit the geometry other than just moving verts between modifiers it won’t work. Next to each button is a help button with further instructions.
UPDATE: Unfortunately, this script only works for Max 4-9. Max 2008 and up are not supported. This is because the iPhysique.gup plugin that it relies on has not been recompiled for the newer versions of Max.