Try this:
$em = $this->getDoctrine()->getManager();
$loggedInUser = $this->get('security.context')->getToken()->getUser();
$loggedInUser->addRole('ROLE_XYZ');
$em->persist($loggedInUser);
$em->flush();
$token = new \Symfony\Component\Security\Core\Authentication\Token\UsernamePasswordToken(
$loggedInUser,
null,
'main',
$loggedInUser->getRoles()
);
$this->container->get('security.context')->setToken($token);