Originally Posted by HarryT
I doubt it. A UK Kindle Paperwhite will only connect to the Amazon UK bookstore, for example.
Note that that doesn't mean that you can't read books from amazon.com - just that you can't buy them on the Kindle itself.
If one Deregisters the Japanese Kindle on the "Manage Your Kindle" page in the Japanese Amazon store, couldn't it then be Re-registered in the American Amazon Store at the MYK page?