Changed a user name in MySQL, but user is still able to login using previous user name
There are several possibilities:


If you edited the privileges manually, you need to execute:

FLUSH PRIVILEGES;


To reload the privileges cache on memory (needed to speedup the checks on every query).
If you edited the privileges manually, you may have omitted an entry. Do not trust the SHOW GRANTS FOR, because it may omit the privileges edited directly on tables. Make sure that there are no user_dev not only on the user table, but on all these.
The fact that there is no user_dev user literally does not mean that a user_dev user cannot login. I will explain myself. The user that you submit is compared against the user privileges tables, so user_dev (let's say that is on localhost) matches:

'user_dev'@'localhost'
'user%'@'127.0.0.1'
''@'%'
'%'@'localhost'


In fact, by default, anonymous users from localhost can login, and all users can read and write on the test database. Run mysql_secure_installation to avoid that.