Field modifications do not neccessarily violate a UL listing. UL likes to use terms like the significance of the listing MAY have been compromised. Only examination by UL can determine that. And then UL will say the equipment is no longer eligible to bear the UL mark, and they will take their sticker back.
An AHJ can approve a field modification or ask for a field inspection from UL. If the manufacturer no longer supports the product, then I don't know what would happen.
It seems here the big question to me would be is the withstand rating or fault current bracing when it's all bolted back together.
We've had old existing switchboards and new sections made by another manufacturer, and there and have accepted almost all without field eval's.
City of Irvine, CA