First, I don't see why this plugin has less than 5 stars and I hope my review puts this plugin at 5 stars. I have been using it since WordPress 3.8 and on WordPress 3.9 it work fines. Plus the security issue has been taken care of - See > here.
Also it has been mad Multi-site compatible. see > here.
I felt to leave a review because it is better than the other username changing plugins and here is why.
1. You can select any User;s Username from a drop down list and then change enter a new username in place of it.
2. The plugin is found under a sensible place Users > Username Changer.
- Other plugins take a main parent place and force their menu item even above the pages and menu... what for?
3. This plugin also adds a change username link in the user table list. So, if you are searching for you user by gravatar by chance I suppose it will be easy as well to locate that user and go directly to changing their username.
There isn't much more to say. This plugin works.
The only thing I can think of that I didn't see this plugin mention is that it checks your desired Username change against already registered users to make sure the name is unique. That is very important, though I haven't ran into that scenario yet.