It's a long and involved process to change a username, and the way vB is built, involves some risk to the database.
Jordan was being swamped with requests for username changes, and he's decided that in terms of the time taken and the benefit to the community, it's not worth doing.
If you wish a new username, you have to re-register. Which is pretty much what Klos was saying.
Remember that if you wish to re-register, you will have to Log Out first.