Man, if I had a dollar for every time I'd had to do this.
One of my major clients has a site policy that if you modify a machine, or relocate it, or do anything to it other than run it and maintain it, you have to bring it up to the latest site safety standards (otherwise, it just has to meet the standards of the day it was installed).
So very often I've got an old scrappy set of prints in the bottom of a panel which I now have to update to show how we've managed to crowbar a safety relay or mini safety controller into the old control system.
You can be sure that, if there's even a logo on the drawings at all, that the company depicted therein is either long gone or has no record of that particular job this many years later. Let alone a complete set of autocad files. And I can guarantee that they have absolutely zero interest in either designing or implementing any sort of safety upgrade to their machine. So, we just have to do the best we can.
Most of the time, we just draw up a one-page "safety circuit" schematic, and red-line the existing drawings to refer to the new drawing everywhere it's needed. A good PDF to autocad converter would give us the option of properly updating things, but we haven't found one really worth the effort yet.
None of this helps the OP, but perhaps we might be being a little harsh to jump on the reverse-engineering bandwagon so quickly...
