If you built a following under a name you regret and want to change it don’t worry, you can do this as of the date this post was written. Take note of the steps below and make sure you don’t screw up. Hold your breath, now go.
1. Create a new account with the name you want (hopefully it’s available) for the sake of trying to keep this simple let’s call it newname.
2. Login to the new account (the one you with the name you want) at Twitter.com and go to Settings and change the newname to /newname123 (You will be asked for your password)
Now log out.
3. Login to the old account (you know the one with all your precious followers) and change /oldname to /newname (without the 123)
Log out.
4. Login to the new account (the one without the followers) that you renamed /newname123 and change that username to /oldname – so nobody can steal it and so that any links pointing to your oldname will not be dead. Make your last Tweet there a notifier of your newname with a link.
I know this is scary, so good luck and …disclaimer…I am not responsible if this doesn’t work for you. It is a simple process that can be complicated if you don’t understand what you are doing. Good luck and contact support (at) twitter (dot) com if you have a problem.