To be fair, it's better than the alternative of making a new account entirely whenever you want to play around with your displayed name, which I HAVE seen and only manages to be even more confusing and difficult to follow.
Having the original-nick identifier available in the profile is enough for general use, and having the feature is validly amusing, but it CAN be confusing when people change very often... you could forcibly fix the problem by adding restrictions to how often you can change your displayed name, but I don't think it's a big enough issue for that.
Or maybe go with Trance's solution and have "original nick" as a displayed field in the side profile? Then again, I know there are some people that have started going by different names than what they were originally registered as (like Moerin and Suikama), so forcing that isn't perfect, either.
Or maybe "previous used name?" That's easier to mask intentionally, granted, and gets accidentally wiped if you typo and have to re-rename yourself, but... that'd probably work best if you want to push it into the main post profile, without being an enormous hassle. Probably.