I'll agree about choice rifles losing value when chambers are altered from originality, but there are a few exceptions to that rule. In my case I didn't mind one bit that my M54 Hornet was altered to K-Hornet because it was done by Kilbourn himself - it doesn't increase its value but to me it doesn't decrease it either. The trick upon reselling it would be to find a guy who feels the same way about it (not that it'll ever get sold under my watch). Therein lies the lone exception to the rule, my personal opinion only. (And it doesn't extend to no-name gunsmith work.)
There were more than a couple great wildcatters/experimenters/innovative gunsmiths back in the 30's-40's whose work paved the way for what we now enjoy today. To the cognoscenti of such stuff their names on barrels bumps values, usually back up into the realm of where it would be if in original configuration. Admittedly the market for these things is definitely limited - the average collector would turn his nose up - but that means all the more for guys like me!
Another example, not as farfetched as you might imagine, would be a Stevens/Winchester/Ballard target rifle from 120 years ago that had its barrel rebored/rechambered by someone like Harry M. Pope, Shoyen, Peterson, Niedner, et al. Such a thing would be exponentially more valuable today compared to if it were bog stock original.
