# Move stats from new user to old user
UPDATE
    tb_user OldUser,
    (
     SELECT
        level,
        points,
        minutes_in_chat_online,
        minutes_in_chat_offline
    FROM tb_user
    WHERE
        username='mikkael') NewUser
SET
    OldUser.level=NewUser.level,
    OldUser.points+=NewUser.points,
    OldUser.minutes_in_chat_online+=NewUser.minutes_in_chat_online,
    OldUser.minutes_in_chat_offline+=NewUser.minutes_in_chat_offline
WHERE
    OldUser.username='mike_1311'
LIMIT 1;

# Remove new user
DELETE FROM tb_user WHERE username='mikkael' LIMIT 1;

# Update username of old user
UPDATE tb_user SET username='mikkael' WHERE username='mike_1311' LIMIT 1;