I would also add that if the modification was simply board-swapping or chip-swapping (e.g., an overdirve processor in a PC), then I think that it would be more permissable to undo the swap. On the other hand, in general, I would leave in place a soldered modification. Indeed, such a mod might have been factory original, as I have seen a number of main boards with cut traces and hand-soldered wires snaking over the board. There will be times when the collector can not tell if the mod was "factory" or "aftermarket".