Basically, once you pick that username, it's yours to keep. If you want a different one, you'll have to sign up again
However, you don't have to be stuck with giving out a MySpace URL you don't like.
Get your own domain for eight bucks a year through our friends at KRDomains.com (we don't get a kickback -- we just love 'em). Then, use their free domain forwarding to point your domain at your MySpace URL.
You'll have a URL that's easy for you to remember -- and easier for you to change -- and you won't have to be forced to publicize MySpace on all of your printed materials. (Remember what happened when MP3.com got sold?)