In the future we will have machines that can assemble food or products on a molecular level. This is not far off. Televisions and handguns and sneakers all downloadable from the internet direct into your hot little hands. Diamond production is at the head of this science, for sure. But that's very different from changing elements, one to another.
These home "makers" will also be able of dissasembling refuse and found matter to harvest the elements required for further production. If you have a huge hoard of high-quality metals, you won't be out scrounging on the street for garbage with copper wire in it to make cool and useful stuff.
Just a footnote on the substituting of non metals for metals.
Airbus has announced it will be using more carbon fiber plastics in the fabrication of its airplanes and less aircraft aluminum. As prices go up, substitutes become more common.