That is not advisable. Some people will abuse it. Bitcointalk should not allow people to individually change their username. Just as Stablexcoin posted already, you can request theymos to change the username for you if you become well established on this forum. Theymos change some people's username recently.
You are absolutely right because allowing such thing in this Bitcointalk forum would going to result to something else. because most people would going to use it as an opportunity to abuse the forum, more especially the newbies, that is when they would realized that the user name they use is not good. To be honest I really like the idea of theymos given the approval first before he or she will think of changing any user name, after all I didn't see any reason of changing to multiple user name here.