Cange my account from UK to USA

Is that because you are in the US .

You can only use an iTunes store if you are in that country

http://support.apple.com/kb/HT6119

Great question, When you have less then a dollar or less then what any app is worth, what we do is something we like to call as Cashing Out the account. This can only be done through Apple and so you will have to call apple and ask them to cash out your account only then you will have an emoty account

If you are moving to the US ,then yes