FOR HEAVEN'S SAKE
(this happened yesterday, but forgot to write)
So I wanted to load up my Wii U eShop money, and to do that on the Wii U eShop nowadays, you have to head to a website and load up money with your credit card from there, using a myNintendo account, which is supposed to be linked to your Nintendo Network ID.
So I logged in there, and loaded 20 bucks on it. Looking back on my Wii U gamepad, no funds had been added. Then I realized I had logged into the myNintendo account that's using my old email address and therefore linked to my old Nintendo Network ID, which I have of course forgotten the name (but not the password) for. Thankfully, you can request a mail from them that says the name of your NNID. So I did so, logged into my old email, and found my old NNID, which is not only now loaded with an extra 20 bucks, but also contains already purchased games like Mario Maker, Splatoon, loads of Metroid games, Yoshi's Woolly World and (iirc) Wind Waker HD.
Then I tried to switch on my Wii U, make a user, connect it with my old NNID and ... it turns out, that since it's already synced with my old, broken, Wii U, I can't link it to this Wii U.
So I looked into what needs to be done and it turns out that to unlink an NNID from a Wii U, Nintendo wants:
1. Name, address, birthday, password.
2. The last 3 purchases from that NNID account.
3. The serial number of the machine you wish to unlink for. (which, in my case, is located halfway across the country)
4. A copy of the police report (?) if the console was lost or stolen.
Great. Fantastic. Enjoy your 20 bucks, Nintendo...