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 name uses the format
described in http://dev.mysql.com/doc/refman/5.5/en/account-names.html.
For example:

RENAME USER 'jeffrey'@'localhost' TO 'jeff'@'127.0.0.1';

If you specify only the user name part of the account name, a host name
part of '%' is used.

RENAME USER causes the privileges held by the old user to be those held
by the new user. However, RENAME USER does not automatically drop or
invalidate databases or objects within them that the old user created.
This includes stored programs or views for which the DEFINER attribute
names the old user. Attempts to access such objects may produce an
error if they execute in definer security context. (For information
about security context, see
http://dev.mysql.com/doc/refman/5.5/en/stored-programs-security.html.)

The privilege changes take effect as indicated in
http://dev.mysql.com/doc/refman/5.5/en/privilege-changes.html.

URL: http://dev.mysql.com/doc/refman/5.5/en/rename-user.html

