All i know, when Facebook username landrush (they called it Facebook vanity URL back then) some months ago, was that once you set a username, you can’t change it. I was wrong. Actually Facebook allow you to change your username.
1. Login to your Facebook account then click on drop down Account. Choose “Account Settings”.
2. And then click on Change like below picture suggest.
3. Then change it to what you like. You will need to check the availability.
4. Save & Done.