If your original barrels are in proof, you will keep the value of the gun even if you add a set of new barrels "by another" some makers of which are posted above.

I did that with a pair of Dickson's, adding steel barrels to use instead of the Damascus originals, said still to be in proof despite rivelling, pits polished out and 20-25 thou wall thickness. Cost some ten years ago were 3,000 sterling each gun. John Dickson quote was 6,500 sterling each gun.

Regards