Both .500Nitro and Huvius offer sound advice - especially, since we're flying blind on this one. Bore condition on any double rifle is paramount in valuation, and may be more relevant to valuation than a conversion itself. The decrease in valuation due to conversion is subjective, but I'd guess 20%. That's strictly a swag on my part and would depend on other characteristics of the rifle, the case, and accessories (bullet moulds, a Davidson telescopic site in Fraser mounts, etc.). I like my Fraser's, and I'm an avid shooter and hunter, not just a collector. I use my Scottish rifles regularly in the field. Even so, a cartridge conversion would certainly prompt me to offer considerably less. Best of luck!