You will rarely, if ever know why the barrels were replaced.
Reeasons: worn out, damaged (anything from run-over by a tractor, blown or bulged by blockage or wrong ammo to left damp and uncleaned until eaten up by pitting and rust), steel made to replace original damascuse, vice versa, long barrels made to replace short or vice versa (more likely), an extra set made and the originals subsequently lost etc etc. The best you can do is guess unless the maker has some record of the reason (unlikely).
Inspect the whole gun carefully, or get someone to do it for you. Current condition, including the new barrels is what you are dealing with now.
With lower priced guns I think it makes little difference to the price. For example a £700 Lancaster boxlock with replacement barrels by Lancaster wouldn't make more if the barrels were original, especially if they have lost some thickness over the years - which is pretty much a given.
With entry-level 'best guns, if anything , the value will rise slightly with new maker's barrels because most people buying sidelocks and boxlocks under £8,000 want to shoot them.
Once you get to the £15,000+ mark, people get more fussy about originality AS WELL AS CONDITION - nobody wants the original barrels if they are worn out but in this bracket, you will get a vv good best sle with sound original barrels anyway.
Once you get into the higher end - say £20,000+ you will find newer guns with original everything and older guns with replacement everything. For example - I have a Kell engraved vintage Purdey on the way to me that was re-barrelled by Purdey in 2001 and re-stocked with exhibition grade wood by a Purdey stocker the same year, totally refurbished and cased with all accessories. It cost the owner £17,000 to do the work. The asking price now is in the region of £20,000. Compare it to the cost of a new Purdey - £60,000+ and it looks like a damned good deal.
