A new update for Miitomo is now available in Japan:
— Cheesemeister (@Cheesemeister3k) March 30, 2016
It also allows you to change your region and link a Nintendo Account from outside Japan:
Miitomo app just updated in Japan, and it allows you to use accounts from regions outside Japan. Signing into my NA Nintendo account now 🙂
— PKMN Trainer Chris (@Chris_BigK) March 30, 2016
Miitomo and My Nintendo launch in the rest of the world tomorrow.