You are required to choose your avatar's class at the start of the game, but can be switched at any time. Tap the Switch Class button from the avatar's profile screen to choose a new class. Your skill loadout will be reset and your equipment will be removed, but your avatar's level will remain the same.
Some skills can be carried over to all classes. If a skill was inherited, that skill will also be shown in the avatar's skill set screen.
Unification and Resets
Your avatar's level and skills will remain unchanged during the season, but will be reset when the Season comes to a close. Your equipment however will not be reset and be carried over to the next Season.