Actually, you wouldnt need to learn hex at all.
the procedure is still complex, though, because i don't know the specifics.
basically, you would use lgptools to find and extract the Yoshiyuki (From battle.lgp), then all of cloud's other weapons. Copy and re-name the yokoshushi the same as all the other swords (So if the Yoshiyuki is aaaf, and the buster swords is aaaa, copy the yushiyuki and name it aaaa.)
then, re-import everythign back into battle.lgp
it will probably take a while to locate and extract all the files, but once you've done that, renaming and re-importing probably shouldn't be too hard =)