You may want to if you're being stalked and you used your real name as your username, and there is personal info attached to your submissions that could help the stalker locate you. Not saying it's likely but it's possible.
But no, you can't. It's the key to all your data (not that this is insurmountable, mind you).
didnt want to change it as I wanted to keep the records of submissions that I had in the past. with a karma of 43...its not high, but i kinda treasure it LOL