Editing Gerber Files? DO NOT DO IT
1) Absolutely right.
2) If you have a PCB that need go out tomorrow and one trace is wrong, editing the Gerber may be the only way. (last Gerber edit: a trace was too close to a screw) - but know the change will be lost.
3) a neat trick: if you NEVER update part designations after your first PCB, ORCAD allow you to edit the layout within limitations using "layout" (I do not do layouts - this is heresay, I have been told to never update part designations)

On the essence of Michaels argument, I want to propose a corrolary to Murphys law: "If you change something by any means other that updating the original shematic/code file it will be lost". Not on this topic, but related to the above: I say "if code is documented outside the source, the documentation will be wrong"

Erik