It didn't just change graphics but it also changed a bit with controls and gameplay. Harry lost his jump punch, has only a single attack and shoots out a projectile. Also all the enemies do touch damage instead of just actually attacking you.
I think Titenic came first though because we've seen parts of it in UMK4's coding (and one of the tracks sound similar), it has credits unlike Harry's Legend, there's unused stuff right in, (although this might be something like SMW with the incomplete version having graphics of enemies in the full one. The only thing is that the waitress in Harry's Legend's tiles doesn't have all her sprites) and the graphics make more sense on Titanic. The reason Harry's Legend is known more is probably the whole MMC3 thing and the fact that (possibly) Harry Potter is way more well known then Titanic.