Two totally different things CD.
Models and materials are two totally different things.

I'm not arguing copyright law. But no, you cannot 'modify' someone else’s geometry and give it away or sell it, unless you are using some extremely rare use of the word modify. If you mean deleting all but 2 or 3 verts and then going from there sure, but you can't load v3 into zbrush, sculpt a nice set of abs, then sell that model. You can distribute the modifications to the model, but not it, nor any derivative there of. That's what morphs are for.

Skies though... aren't anyone’s.