[Arikado]

I don't know anything about that, but my logic tells me that there's no difference with the exchange rates no matter where you are in the world, unless someone is blatantly trying to rip you off. Just change your money before you go. Besides, even if you're fluent in Japanese, it's always easier to speak to a banker in your own language right? ¦3

Edit: Oh, the thing about the fee for exchange. If it costs nothing to exchange money in Japan, then by all means, don't let yourself get charged for that. Then, as you said, just change it back free of charge when you get back.