Back up revision level and add "Obsolete" to the document description field

I have a C# server method that creates a new revision of a document then after some edits, moves the document to the "obsolete" state (not CN controlled).

  • I would like to add script that will
    • back up the revision level one increment (back to what it was prior to the new rev being created).
    • add "OBSOLETE" into the document description field

Can anyone help me with this?