If you look in a strategy guide for the game, they will recommend that you beat Kary (or Marilith, on PSX) before getting the class change.

I disagree, however.

After beating Lich, you can head for Crescent Lake, get the Canoe, head to the Ice Cave, get the Floater, head to Ryhuken (Lyuken, on PSX) Desert, get the Airship, then go to the Castle of Ordeals (or Trials, on PSX), get the tail, then give it to Bahamut, get your class, change, then head to Gurgu Volcano (or Mt. Gulg, on PSX), and beat the hell out of Kary with your new shiny sprites.

And, sure, you can beat the fiends out of order, like beating Kraken or Kary last, but why would you want to? The storyline makes a lot more sense if you beat them in order, and plus, they get harder as they go, so if you fight Tiamat before Kary, well, Kary will be too easy, and she's fun to fight because you need strategy to do it.

Feion