Official requirements?
The only official rule we have about username change is that "You can either donate 50 BTC to become a VIP or PM theymos asking to change your username (although he rarely accepts such requests nowadays)."
Wow 50 BTC that's alot, i thought they might have reduced the amount of BTC paid to become VIP , especially now bitcoin price has increased to a nice rate . Base on most users comment I don't think changing of one name would be easy at all , but I guess the reason why is like that is for safety reasons.