Few users have been granted that request by theymos and we all knew that reputation is also part of this decision and if you are an established member of this forum. However, some exception could be made like this case:
Yes, a change from alia_armelle to alia is a minor change which I would normally do for anyone who can convince me to spend time on it. (If I recognize your username, I'm much more likely to handle your requests. I'm also much more likely to look into your scam accusations...)
I never accept payment for this sort of thing.
Likely its a case to case basis and depends on theymos judgement.
But like others point of view. Changing username could do some confusion and harm to such situation like potential scam deal