Hi, I read that you can make a blendshape without duplicating from the original model.
I've already skinned a model and found that I can't detach a bit of skin and reattach it because it asks me to put it back to bind pose, which I can't do because it's already fully rigged with IKs and everything.

So in order to avoid this, I have imported a previous unbound model and used it as a blend shape. But it also adds some extra unpredicted deformations along with the desired deformation. I deleted history and everything from the imported model and they have the same number of vertices and everything.

So am I missing something?