If you torrentZip the set you already leeched from UG you can update it with the new set uploaded there.
You just have to download the new torrent, start it and immediately stop it.
Copy all the roms from the old set into the new SNES folder your Torrent client created.
Run "Force re-check (or whatever it's called)" on that new folder.
It should then check what games you already have, once that's done you just start the torrent and it'll download the new games.