I don't see that modification having much, if any impact on a gun's value if done correctly. As regards Smith guns 1) we no longer have the order cards so there's no way to know what original chokes were ordered, 2) with rare exceptions choking is not marked on a Smith gun, and 3) most shooters find the more open chokes much more practical for actual shooting. The only exception I see to these conclusions are those rare Smith guns that do have a choke stamp on the barrels; I would think changing chokes in those instances from what was factory stamped on the gun would certainly impact the value of such example to the collector.