A child blueprint is a copy you cannot fully modify but that you can somewhat overwrite. An Instance essentially.
A copy (as in ctrl-c ctrl-v) is a copy you can fully modify and change skeletal mesh to while keeping other Non Animation functionality.
The Anim BP is linked on the Mesh within the item Details.
you will need to either duplicate the anim BP as well and modify it - if you changed the skeletal mesh - or simply link the animation blueprint again.
As far as the mesh falling to pieces, that’s probably a feature of the blueprint you copied. Hard to tell since we don’t know anything other then what you wrote above…