The WoW ToU (Terms of Use) was recently updated so that anyone playing outside of the territorial boundries of the United States, Canada, Australia, New Zealand or Singapore is now in violation of the Terms of Use and can be banned at any time.
So, if you are intending to move into one of the above countries then i see no problem playing on a US server. You will experience higher pings until you make your move but we have a rather large Euro contingent that plays with (relative) ease.
Also, given your current Euro server characters i would most definitely switch to a US server as you will be losing nothing but gaining better playability as the characters you currently have really are not that big of a loss. Level 20 takes really no time at all once you are familiar with your surroundings and know the general scheme of things.
Hope that helps :)
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