Each user has a display name, which is the only name ordinary users see, and a username, which is what you use to log-in with. Almost always, they are the same.
If I change a name for reasons of appearance, then I only change the display name. The username remains reserved, the user can still login using the username, PMs can still be sent to that username, "search member" allows searching for the old username, etc.
If I change a name for reasons of privacy, then I change both the username and the display name. In this case, it is possibly hazardous to reserve the old username, since it allows someone to test for the existence of that username, possibly defeating the privacy benefit. If you're worried about being impersonated, then simply don't seek to have your username changed.
OP, this is solely my opinion, but I highly believe that in the remote possibility that your username change request is accepted, it will only be made at a "soft" level - as in, due to a reason of appearance as theymos puts in the quote provided by tranthidung. In most places that I browse making a deep change regarding a user account is something that happens very rarely, and I personally never seen it happen - the only reasons I see as plausible would be to keep someone safe / privacy issues such as theymos said.
In your case, however, I see few arguments that could sustain such a quick username change considering that you've created your account on the 1st of October and then 8 days latter you suddenly though that you may not like your CHOSEN username. I imagine that the few users that ask this change to be made probably don't want to start from 0 regarding their rank / merit, so I somewhat could understand their eventual request, but not on your case.
Considering the time that a mod / theymos will have to employ to actually change your username (no matter how small it is) vs. you creating another account and start using it, I see the latter option being the quickest for your situation - I'm sure mods / Admins have far more greater issues to deal with, and they'll probably throw your request far into the queue of "issues".