Technically, it makes sense that if you have your AdSense account under your personal SSN and then you start a business and want to switch that account to the business - you will need to open a new account. Why? A business is its own entity, just like you and I are our own entities. My business has its own tax ID number, I have my own social security number. I assume the same logic applies when moving to a different country. If you become a citizen of that country and have your taxes taken out there, then the same logic applies.
Hence the line in the AdSense FAQs on Am I a Business or Individual account?, which says; "Should you ever need to select a new account type, our specialists will work with you to close your current account and to open a new account with your updated information."
Forum discussion at WebmasterWorld.