Yup. First log-in to your SportsCastr account on either the iPhone app or at SportsCastr.com on a desktop/laptop computer, and click on the Settings icon - which looks like a gear. (If you're in the iPhone app you will first need to go to your Profile and then click on the Settings icon.) Once you are in Settings, click on "Edit Profile" and you will be able to change your Username.
Remember to click "SAVE" at the bottom of the screen!
NOTE: Usernames cannot exceed 15 characters and may only contain letters A-Z, numbers 0-9 and underscores ("_")