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

The RENAME USER statement renames existing MySQL accounts. An error
occurs for old accounts that do not exist or new accounts that already
exist. To use this statement, you must have the global CREATE USER
privilege or the UPDATE privilege for the mysql database.

Each account name uses the format described in
http://dev.mysql.com/doc/refman/5.6/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/5.6/en/stored-programs-security.html.)

The privilege changes take effect as indicated in
http://dev.mysql.com/doc/refman/5.6/en/privilege-changes.html.

URL: http://dev.mysql.com/doc/refman/5.6/en/rename-user.html

