Read this first https://wordpress.org/support/topic/better-plugins/
Plugin to change your default admin username ( with GUI to change all other admin names too ).
Tested on multisite
Will check for:
* Empty names
* Existing usernames
* Valid username ( no funky special characters )

As always -> make a backup first

I have a lot of questions and I want support where can I go?

http://pluginsupport.mijnpress.nl/ or drop me a tweet to notify me of your support topic over here.
I always check my tweets, so mention my name with @ramonfincken and your problem.

3.2.1

Changed: Possible XSS bugfix reported by CBiu

3.2

Changed: removed mysql_real_escape_string

3.1

Added: Extra check to prevent empty username

3.0

Added: Multisite support. Thanks to Peter Luit for reporting!

2.0

Changed: WP standards used to get all admins
Added: Extra security check based on 'remove_users' capabilities

1.1

First release

1.2

Tidying of html code, added screenshots

1.3

Applied WP way of handling usernames

1.4

Same as 1.3 Commit error so scipped 1 version

1.5

Added links to plugin page & some extra security

1. Upload directory admin_renamer_extended to the /wp-content/plugins/ directory
2. Activate the plugin through the 'Plugins' menu in WordPress.
3. Visit Plugins menu to change your admin name(s).