Each class can get promoted in Job Center in Twin City (402, 389) when reaches level 15. Each class will get a different title and enhanced attributes after the promotion. Character can learn new skills or train old ones.
Different NPCs are responsible for promoting each class.

Name	Class
WindSage	Ninja
WarriorGod	Warrior
ArcherGod	Archer
TrojanStar	Trojan
TaoistMoon	Taoist

For the details, please refer to: http://co.91.com/guide/classes/intro.shtml

Character promotions have a level requirement. Accordingly, titles and attributes will be changed; and new skills will be available too.