Not to mention that shot like Tungsten-Matrix from Kent will make the shotgun perform the same way it did for your Grandfather, back in the day. It makes it hard to go back to steel shot.
There was a good episode of the Orvis Hunting and Shooting Podcast a short while ago entitled "New and Contemporary Side by Sides, with Greg Carpiniello" where the host and the manager of Orvis' gun room discussed just that...new and contemporary side by sides. Definitely worth the listen if you're thinking that you would like to hunt all your flying game with a side by side.
You'll also want to consider the fact that you won't want to hunt walked-up birds with the same gun you're using for waterfowl. My Fox A Grade is choked IM/IM and would be a poor choice for walked up birds but just right for waterfowl using Bismuth or Tungsten-Matrix. Something lighter with more open chokes, say IC/M, and shorter barrels would be better for upland birds.