[MMM]

You wouldn't have to "switch", but if you were to open an account at Citibank or HBSC you wouldn't have to open a bank account IN Japan.

I also have a US Bank account, and I can use my US Bank account ATM card at CITIBANK ATMs. I think there is a small service charge, but if you could find out if your present bank card works at Citibank machines, then you wouldn't have to do anything (assuming you were close enough to a Citibank machine and didn't mind the charges).