Nintendo Network Account Can Only be Transferred if Your Wii U Breaks
And you'll have to send your console to Nintendo
In doing away with friend codes, Nintendo has taken significant steps towards having a cohesive online system, but it's not there yet. While you now only have to add a friend once, instead of again for every single new game, your Nintendo Network ID is still not quite as sturdy as your PSN or Xbox Live account. In particular, your Nintendo Network ID is essentially limited to one Wii U, meaning you can't log in on a friend's console or, so we thought, transfer it to any replacement you might need to buy.
From the Wii U support on Nintendo's website:
A Nintendo Network Account can only be used on the console where it was created. In the future, you will be able to use your Nintendo Network Account with future Nintendo consoles and other devices, such as PC's.
Once an NNA has been created, it can be deleted from the system. However it cannot usually be transferred to a different Wii U.
But now, Nintendo seems to have changed its tune, or at least clarified on that “usually part”, after a bit of pestering from Kotaku. In a statement, the company said:
Nintendo of America's repair process includes the transferring of the Nintendo Network ID to a new Wii U console if a replacement system is needed.
So while you still can't log into your Nintendo Network Account to access your content and friends lists on the second Wii U you have in your holiday cottage, secondary caregiver's house, etc., you should at least be able to recover stuff that's linked to that account if your console breaks, though you'll have to send the old one to Nintendo to do so.