It works ! Thanks NinStar

My Wii U user
was not deleted, unlike the unlucky guys above. That's unfortunate

.
So how I did it?
- Create a second user and log in
- Install FTPiiU Everywhere if it's not install on your Aroma env.
- Press
L + D-pad Down + Minus while staying on the Wii U menu, you are now on Wii U Plugin System.
- Search for ftpiiu then press
A
- Make sure that "Enable ftpd" is on
true and "Access system files" is also on
true (it's very important)
- Use Filezilla on your PC and type the ip address and the port shown on the ftpiiu menu.
- Connect to your Wii U (Make sure that your WIi U uses the same WiFi as your computer)
- Search for your
account.dat
- Open with Notepad (or alternatives) and copy paste what NinStar gave after the line "MiiName"
- Save it and make sure that FileZilla saved it by reopening account.dat
- If it's done, disconnect and close FileZilla.
- Leave the Wii U Plugin System, the Wii U will restart automatically
- Change user account and you will now see that NNID has disappeared from your Wii U user.
If the steps are unclear, ask me for more details. I can make a video if you want...
I also backup my account.dat before doing any modification, i don't know if it's useful but do it.
So now, I'll be using Pretendo Network.