Here is what I did :
I just created many meshes right after importing my images from photoshop before starting to actually rig and animate.
During animation, I realized I wasn't happy with one of the meshes, that's why I'd like to make it a simple region attachement again (and maybe create a new mesh from scratch) !
Note that I haven't animate any of the vertices for this mesh, I just created new vertices in edit mode of the mesh in setup mode, but without actually any ffd.