Well Nintendo support gave me this method to unlink nnid without formatting, never heard of it before.
1. Go to the system’s HOME menu. Select “System Settings,” then select “Nintendo Network ID Settings.”
2. Select “Sign In.” Enter the password associated to the Nintendo Network ID, then “Confirm.”
3. The error message (“You cannot use this Nintendo Network ID on this console”) will appear on the screen.
4. While the error message is onscreen, hold down the X button and Y button, and tap “OK.”
5. Select “Yes” to unlink the Nintendo Network ID from the system.
6. You will receive a message that the NNID link has been removed. Tap “OK” to return to the system settings.