It is a judgement call and no one will back you on it. You go to an older house you replace a recept on NM with the under size ground. Rule of thumb is that it doesn't matter what code it was wired under, when you put it back together, it has to meet today's standards. How do you tell the customer that you can not use the old wire? Who is going to back you with the customer? Who is going to back you if it fails by no act of your own? It's a case of damn if you do, damn if you don't.
I would suggest to anyone that if they are not sure what to do, talked to the AHJ.
I typically identify the problem, recommend that it gets replace that usually goes no where and include my comments with the bill. Gotta CYA.
