Before you turn your AMEX into a Japanese AMEX, I would also recommend that you check the credit limit they will give you.

For the first months after I received my Japanese credit card, the limit was far too low for my needs. I reached the limit practically every month - and fell back on cash-payment (or my UK credit cards) to keep me going. When I asked them to increase the limit, the first person I talked too was terribly bureaucratic - told me that I had to wait a year. It was a bit of a battle to get a card that was actually useful.