When do i change class?

[Anova]

Ul'dah's Most Wanted is the class quest to complete if you started as a gladiator. After you finish this quest, the npc (Mylla) will suggest you take a look at the other guilds. You can visit the respective class guilds and complete their quest to unlock the class. You can do this for all 9 combat classes, 8 crafters, and 3 gatherers, and switch at any time you wish between them.

So you can spend 1 hour leveling gladiator to 33, craft yourself a suit of armor with armorer in 10 minutes, make some money as a miner for 20 minutes, and then go back to gladiator for a FATE you happen to see while mining.

If you wish to become a paladin, you will need to first travel to Gridania and complete the Conjurer's quest, My First Cane). You don't need to do the class quests for this class, although they are heavily recommended as it provides you with equipment, and abilities (including cure2 at lv.30). For the time being, you only need to level Conjurer to 15 and Gladiator to 30.

After you finish the lv.30 gladiator quest, Rematch, and have a lv.15 conjurer (no quest needed, just the level), you can access the Paladin quest, Paladin's Pledge.