So do you want a new document in drafts or move the document from Approved to drafts?
If you want a new document, you could create a rule that copies a document post save from the approved folder to the drafts folder. Only issue I see there is that you would have the new version in both spots (ie. version 1 was in the approved folder, edited and saved back as version 2, now copy and if someone looks in the 'approved' folder, then they will see the edited version by default not the version 1, or the approved version (not sure if that sentence even makes sense!) There are ways around this, but would take some custom programming.
Hopefully you just want to move the doc… much easier. Just create a rule that runs on update and moves the document back to drafts when it is changed… again, the only thing is that update fires if you change content or attributes, so even if someone just changed the name on the doc, it would be moved…
So if you can give some guidelines on exactly the behaviour you are looking for, might be able to better provide some input.