You should be able to update the email address on your existing account via http://appleid.apple.com or via your computer's iTunes (Store > View Account menu option) - if you create a new account then all of your existing downloads will remain tied to your old account, you can't link accounts and/or transfer content from one account to another account.
If you do update the email address on your account, and if you have an iPhone, iPad, and/or iPod Touch, then after updating it you might need to log out of your account on them and then log back in with the updated version to 'refersh' your account on them.