The Quick Change system allows you to quickly change between your characters’ Main Class and a Sub Class that you’ll choose at level 30 from the quest (Goddess) Brand New Power given by Clarisse in Sacred Springs. Your new Sub Class will start out at level 25, and come preset with new class-related clothing, a new Divine Core and keys, a new weapon, and class-related skills. The Sub Class has its own combat level, HP and MP, equipment slots and shortcut bars independent of the Main Class. You switch between classes using Exchange Points, also known as EP. You can change between the classes by the 'z' key with 3 seconds between each change.
The exchange points you have available are near your status bar, where each EP is visible as a colored heart. You can refill your used EP by being in battle and it will light up your bar. Once the bar is full, it will automatically convert to a filled EP heart. If you change your class within a city, it will not require the use of EP.
The experience required to gain a level for a sub class is half what a main class requires of the same level.