Nope — what you sign up with is permanent, so choose your username wisely. :)
However, if you want to create a new account with a new and different username, you totally can! Just be sure to use a new email address during the signup process.
Some additional info to keep in mind: Your username must be 3-30 characters with any letters, numbers, or symbols -_$* (but it must have least 1 letter!)
If you have any questions, please send an email to firstname.lastname@example.org and we will get back to you as soon as possible.