Menachot 32 - How to Write a Mezuzah
If a Torah scroll wore out it may not be made into a mezuzah by cutting out the two required paragraphs, sewing them, and affixing them to the doorpost, because we may not lower objects in sanctity but only bring them up. So lowering in sanctity is the only problem, but if not for that, we could? What about the requirement to write the mezuzah on the outer, tougher part of the skin? - That is only the preferred procedure.
In addition, mezuzah requires scoring with thin lines before writing, and it does not have to be copied from a text, because everyone knows it by heart.
Art: Johann Hamza - An Old Man Reading
