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'.
If you specify only the user name part of the account name, a host name
part of '%' is used. For additional information about specifying
account names, see [HELP GRANT].

URL: http://dev.mysql.com/doc/refman/5.1/en/rename-user.html

