I don't think it's possible and the only option is to create a new subscription. I had the same problem. When I signed up for a subscription I was in the US and then I moved to India. I tried to change the credit card information (from a US credit card
to India credit card) but the portal did not allow me to change the country in the billing address.
I ended up raising a support ticket for that and they told me that it's not possible. Because of "legal" issues, country on a billing address can't be changed. I left that at it and ended up creating a new subscription.
As far as moving VMs are concerned, I think it's much more easier now with asynchrnonous copy blob operations. Do take a look at my answer on Stack Overflow regarding moving stuff from one subscription to another:
http://stackoverflow.com/questions/12274645/transferring-all-data-from-one-subscription-to-another/
Hope this helps.