I doubt it impacts value much. The fact of the rebarrel will more likely do so than dovetail vs chopper.
You don't say how old the guns is. It's a little odd, I suppose, that new replacement barrels made 30 years ago would be dovetail lump, but the owner might have chosen dovetail because the original barrels might well have been dovetail. Long after chopper-lump became common, many best quality double guns from the very best British makers continued to be built with dovetail lump barrels, because their best barrel makers (older guys) had been trained on dovetail. Plenty of pre-war best Purdey's, Holland's, etc still around with original dovetail lump barrels. Although they use chopper-lump today, Westley Richards best quality pre-war double rifles were usually shoe-lump. New built "best" double guns today are almost always chopper-lump, but that doesn't really apply to pre-war British guns, especially to those built before WWI, and it really has little to do with the quality standard or the value of the piece.
