You have to remove the previous role first, then add the new role like
$u = new WP_User( $user_ID);
$role_of_user= $u->roles[0];
$u->remove_role($role_of_user);
$u->add_role('new_role');
If user has more than 1 role then
$u = new WP_User( $user_ID);
foreach($u->roles as $role){
$u->remove_role($role);
}
$u->add_role('new_role');