Sharing Rigs with the same blendshaped mesh

I have a character that has many body morphs. All is well until the morph deforms the character beyond what the rig supports. As an interim step I thought to simply go back to the modeling tool and "Bake" the morph in and import it into unity and grab the new rig, since unity would create a new rig that will work.
Then I thought would toss (or at least not use) the baked version of the mesh. But I have never been able to replace the avatar from another prefab. Even though I set the root bone. and has thes ame verts.
Is this not possible? what am I doing wrong?

This is also true if I want to just create morphable clothing as a separate prefab and expect to drag them out of a prefab into my main character. It doesn't work even if it was created exactly the same way as the main prefab. Setting the root bone has no effect. Only solution is to duplicate the bones and literally just put that "on top" of my main character. A huge waste of resources....