The only currently sanctioned method of changing your username is to request your current account be deleted and then create a new account with the name you want.
Of course this means you loose all points status and levels.
In regards to the rules, It is likely the forum software supports this, but if it were allowed, then the admin and/or moderators would be spending a large portion of their voluntary time changing usernames. If you haven't noticed there are more than 570,000 registered users. Even if 1% of those users wanted to change their username it would be 5700 username changes.
FYI. The majority of the rules on any forum are a direct result of abuse due to a lack of rules to prevent certain actions. Things like cross-postng, multiple accounts, advertising, w-a-r-e-z, cursing, being a posterior-orifice to other members, etc would be the forum norm. Hence the reason most of those old unmoderated usenet newsgroups have no active contributing members. Only those that are strictly moderated still have activity (last time I checked a couple of years ago).