If that's your only worry you can update the ink every ten or so years, verify that the newly created paper works, and that should do it. Of course, that's assuming that stays your only worry. I wouldn't convert to metal, at an additional expense if that's your only concern. It doesn't even have to be ten years, it could be less. It really does depend on what you want to do, but changing to metal seems quite extreme if you don't need that. If you think that it could be exposed to certain elements, I'd say go for it.

You should probably have multiple backups in different places anyway, which could actually defeat the point of metal backups, as long as the paper versions are all secured.