If it's a really old sgian, you could always consider making up some colourful story, perhaps in the form of an old legend, to explain the scratches... as per the aforementioned Antiques Road Show that would probably increase the resale value from a theoretical million to a theoretical 2 million.

But seriously re changing the blade, I support the postings that said it will trash the value. Not to mention things like the intrinsic heritage.