how can i change my username?

[awesome31312]

And this is where you'd use the business portion of your brain and evaluate:

Should I spend $28,749.50 to change my Username on Bitcointalk? Or is it cheaper to create a new account with the username of my choice instead?

And what if your name is your brand and you dont want anyone to misuse it and someone has already registered it? can it still be regain by becoming vip or donator?

That's where intellectual property rights would come in, you can file a copyright claim in that case.