As long as you have the lowest level available this should still be an option. I have done this many times in the creation of the morphs for rex and roxie. If at any point you delete the lower leves or you export a higher level or a localised subdivision you are correct that you won't be able to do that.

 

Higher poly models are great. They have a purpose just like low poly models.  Both are valid and both can be morphed into whatever you want.