Character Upgrade

[Raziel]

So, I'm playing FF1 on PSX (on normal) and I have a question. Right now I'm in Crescent Lake and I just defeated the Marilith. When my characters advance to the next state or smth? Like Warrior --> Knight and etc.

[Crossblades]

Once you get an airship, you can head to the Citadel of Trials to get the Rat's Tail and then to the Dragon Caves to give it to Bahamut to change classes.