I heard that Kuja's theme from FF9 is a remix of Tyrano Lair, which is a remix of Cursed Earth from FFV, which is itself a remix of another song from FF3.
This is what I've heard, but when I listened to all the named songs on Youtube I couldn't hear them all the way through. It's important to remember that Tyrano Lair was one of Uematsu's handful of songs, not Mitsuda's, so it has more of the stuff he puts in his songs.