The Problem with WordPress is that, it does not allow you to change your usernames. When you want to change it for some unavoidable reasons, you will be left with no option to do that. It can also lead to some security issues. To save yourself from this issue, you can use this plugin.
WP-Optimize helps you to change the username of any account in your WordPress installation. It also helps you to delete post revisions, spam comments and optimize your WordPress database.
You can download it from here or search for WP-Optimize in your WordPress Plugin Manager.
With the help of this plugin I deleted 966 Post revisions and 5,661 Spam comments.
Try the plugin and share your views with us.