There are two times in a game when a character can adopt a new role. One is at level 25, when a character initially chooses a role. At that time, all attributes and mastery points are reset, and the player may redistribute them. There is a second opportunity to change your role, more commonly called a restat, and that can be used any time before the character reaches level 70. It works the same as the first role selection, but if the character changes roles, they lose all skills that are not common to the roles. Unfortunately, after you use that second restat, you cannot change your role again. This may change when advanced clans are released.
Additionally, one can change their role by using a "role changing pill", bought from other players or obtained by rare chance from a Blue Dragon's Treasure Box.