Ninty has a record of which console is a NNID attached to, you can remove it from the console nad put another one, but the one removed can only be used on that console. What calling Ninty does is unlinking the NNID from the console so it works on the SOurce 3DS
So just to be clear, I can use Godmode9 to remove the NNID both on Source and Target 3DS's if I want, and then I CAN create a new/login with existing NNID on those without problem? But the NNID that was removed just can't be used in any other console? What happens to the NNID that was already in the target 3DS?