No. It is not possible to change the username you selected during registration. However, you have the option of registering under a new username (see also: How do I register?). However, be aware that the email address you use for the new registration cannot be the same as the registration for your 'old' username. If you want to use the same email address for your new registration, you must either delete your old profile (see also: How do I delete my registration?) or change the email address used on your 'old' profile (see also: Can I change the username I selected during registration after activating my profile?):