When you initially create your account, when you fill out where you live, it takes that info and through this info it will give you your appropriate realm. So if your address is in the UK, you get the EU servers. If you live in the US you get the US servers, and so on. When you upgrade your current account with BC it will probably keep you in the EU realm.
Either way you look at it, you will have to start over fresh with your characters, unless you pay to transfer them. I'm not sure if they would do that or not.
Also, they may not let you do this and will make you buy another copy of WoW, so who knows. The only way to find out is to ask I guess.
**Note** This is all speculation and not concrete info. I've seen similar questions on these forums and the info I just gave was the info I got from those other threads
**Note**
Edited, Dec 8th 2006 1:35pm by jpaugh