Finding out what you need to buy before you buy it = money saved in the long-run.

And as far as I can see, you can't get a US account from an EU code, yet you can get an EU code from a US account. How the hell does that make sense? People would be more likely to do the former rather than the latter.

http://www.wow-europe.com/en/support/account.html

It has a thing there about what to do from going US to EU, try contacting them and ask about the reverse.

But if you can't do it, it's your own damned fault.