RENAME USER

CREATE USER 'newuser' IDENTIFIED BY 'mypassword';

Query OK, 1 row affected (0.02 sec)

SHOW GRANTS FOR 'newuser';

+-------------------------------------+
| Grants for newuser@%                |
+-------------------------------------+
| GRANT USAGE ON *.* TO 'newuser'@'%' |
+-------------------------------------+
1 row in set (0.00 sec)

RENAME USER 'newuser' TO 'testuser';

Query OK, 0 rows affected (0.08 sec)

SHOW GRANTS FOR 'testuser';

+--------------------------------------+
| Grants for testuser@%                |
+--------------------------------------+
| GRANT USAGE ON *.* TO 'testuser'@'%' |
+--------------------------------------+
1 row in set (0.00 sec)

SHOW GRANTS FOR 'newuser';

ERROR 1141 (42000): There is no such grant defined for user 'newuser' on host '%'