I saw the first one. I probably should've made the distinction between 'replace' and 'alter', because models CAN be swapped, but not outright replaced with fully custom models.
As for the second one, I've never seen that. In all likelihood, it's just a vertex/texture edit. It's POSSIBLE to edit vertexes slightly (position, rotation, UVs, etc.), but it's not possible to replace the model outright, as the vertex indexes have to match, or you'll just end up with a very corrupted-looking model.
Obviously, nothing is impossible, but I'd advise to not hold your breaths yet. Ohana3DS Rebirth development is pretty stalled and there's no real interest from skilled developers to put forth the effort for something like it.
It's sad, but I guess that's how it is right now.