condition, condition, condition. it all boils down to the condition of the shotgun you are wanting to convert and value.
if the shotgun can be restored to it's original condition easily, it might be worth much more money than if it were converted to a trench gun. if it's in fairly pristine condition, personally, i'd leave it be, and find a less than stellar example to convert. again, it comes down to condition.
if it's beat all to hell, rusty and looks like dog excrement, and there is no personal sentiment attached to the shotgun, then it might be worth converting.
only you can make that decision as to whether to convert or not.
hazarding a guess and an opinion, my thoughts would be if the gun were restored to it's OEM factory condition, it would be worth much more than one converted to a trench gun. I doubt it would ever be worth converted to a trench gun, as an original from the factory trench gun. it would just be a replica at best. that is strictly a guess and an opinion on my part though.