if you haven’t changed the polys or vert orders in any way you should be able to store a morph at any level. I had some fun sometime back taking a base head and upping the subds and did a little detailing then stored a mt at the highest level, saved the tool. then reloaded the original low poly one cloned it a few times and changed their basic layouts…for instance one I took a row of verts and pulled them down into the head so it looked like someone took an axe to it…and various others and saved them as objs.
then reloaded the subd’d mt one and imported the different base meshes in for kicks to see how it changed the look of the highest level…made for some very interesting characters…was lots of fun to do.