Unless they're changing the system even more come Jan 6th you will be able to access your account there given that SE is an international company, the only restriction is account type where you can't register a NA/EU copy onto a JP account and so on.

You may however get hit with an account lockout since you'll be logging in from a completely different ISP let alone IP which you will then have to reset your password, but that's about the only thing I can think of since I've played XIV and XI with the same SE account in Japan and America, but both of my games were JP version.

You can't pay for the Japanese account with US credentials and vice versa if I recall (the only other issue I can think of)..but after all, come January 6th they're doing changes to the SE account system to add in additional payment option, so things may completely change next week in terms of this situation.