All Warrior class change quests are available from Maskman in Warrior School in Amakusa.
You can take the Warrior Class change quest from Maskman when you are level 10.

Upon reaching level 30, you can take the second class change from Maskman to become either a Paladin or a Berserker.

Links are below to the relevant pages for those class changes.
Paladin Class Change
Berserker Class Change