Fred: check the "Should I lengthen the chambers in my 20 gauge or 16 gauge L.C. Smith shotgun?" post under the Design and Engineering section of the FAQs
https://lcsca.clubexpress.com/content.aspx?page_id=274&club_id=43784

It the gun is a shooter, and the work is done by an expert, maintaining adequate end-of-the-chamber wall thickness, it probably won't decrease the value and might make the gun easier to sell. I'd keep the work receipt to document who did the work.

Done wrong the barrels could be ruined; a bulge at the end of the chamber from "inexpert" chamber lengthening