Finally I found the solution. I had to make a new security token and set it as security context.
$user = $this->getUser();
$user->addRole('ROLE_ADMIN');
$this->get('fos_user.user_manager')->updateUser($user);
$token = new UsernamePasswordToken($user, null, 'main', $user->getRoles());
$this->get('security.context')->setToken($token);