Also, per the rules of QQ, total account deletions are not done. They would have the potential to upheave threads with massive amounts of information, and disrupt other things. While individual posts can be altered or deleted (the latter needing a reason, though generally lenient in practice), trying to erase everything won't really work.
On the plus side, if you simply want to change your username or anything in your profile, it should alter those details everywhere in the website, so that it will seem as if you were always posting with your current username. At least, I think that's how it works.