ROCK FLAG AND EAGLE!
Microsoft has, at long last, made account migration an easy, automatic process. For years, folks who moved to new territories, like myself, were stuck with their Xbox Live accounts tied to their old homeland, unable to use any new credit cards, addresses, or local points cards.
Now, you just have to go online and use the account migration tool to select your new home -- after going through a few simple online security hoops, of course.
Microsoft warns that some services previously available may not be once you move, and that you should download anything you definitely want to keep. Your Gold membership status, Gamerscore, and account details will all be preserved.
Great news. I am now, officially, recognized by Xbox Live as a resident of the United States ... and it only took over four years.