I have a paid European WoW account, but would like to play with friends on a US server. This site claims that it's possible by editing a line in the realm list file, but that didn't work for me. Other sites claim that it's impossible. Is there a way to switch between EU and US servers with a single account?
No, there's no way to transfer an account to a different region. Sorry. :(
realmlist.wtf file as that link describes will make your game client connect to the US servers, but it doesn't change where your characters are stored or what type of account you have. The only reason you'd want to do that is if you had both a EU account and a US account, and wanted to switch between them without installing a second copy of WoW.
No, your account is tied to a region. The realmlist.wtf file is just for the purpose of getting the realms from a different region. So lets say you have created an US account, you would only have to change the realmlist.wtf file to change the login servers (and realm servers) to those from the US.
Actually this is possible. If you create a US account and buy a subscription for that account you can transfer your characters to that account and play over there. But the downside is that with this thr region transfer for your characters costs almost half the game did by itself and there is also a glitch with this where you can lose your armour and money. My friend had done this and notified me about the bug before i did the transfer myself. Although it only happened to him once.