Syntax:
RENAME USER old_user TO new_user
    [, old_user TO new_user] ...

The RENAME USER statement renames existing MySQL accounts. To use it,
you must have the global CREATE USER privilege or the UPDATE privilege
for the mysql database. An error occurs if any old account does not
exist or any new account exists. Each account is named using the same
format as for the GRANT statement; for example, 'jeffrey'@'localhost'.
The user and host parts of the account name correspond to the User and
Host column values of the user table row for the account.
