Ayrton, I appreciate your concern. Yet, I'm not sure I follow your argument.
First of all, even UL will not say something 'voids' the listing. The most UL will say is "we don't know." In many cases, they will say "sure, go ahead."
For example, the replacement inserts made by Cutler-Hammer are specifically listed for replacing the insides of other panels. It's a legitimate product.
Second - as in the job I did - it's pure silliness to assert that replacing the damaged innards of a Brand X panel with new innards from another Brand X panel could in any way affect the listing, or safety, of the panel.
Finally - again as in my instance - it's not always possible to replace the panel. Or, at least, not possible without major construction efforts and a prolonged shutdown of a business that really couldn't be shut down.
As for the panel being "old" at twenty years .... it's been my experience to rarely see equipment that new! I just came from a place whose equipment was old when Eisenhower was president. When my own home was wired, Hitler was still planning to invade Poland.
