>>390835948
You can still copy over a lot of stuff depending on how complex your changes are. Usually it's sufficient to drag a new FBX file into your scene, and copy over the old avatar descriptor. Don't forget to reassign the "Body" mesh in the visemes and such.
Pumkins tools can even automate most of this, and it can also copy over physbones. The only "hard" part is putting any props you may have on your fresh avatar, but even that is as easy as just reparenting them from your old hand to your new hand. Plus, Pumkins can often do that too.

Never unpack your prefabs. Even if you need to reorder bones for the IK, you can go to the "Gameobject" menu at the top and set a gameobject as first/last sibling without breaking the prefab.