I unlinked it with Nintendo before. It took 5 days for this to work, but it worked. And I'm pretty patient.The tricky point is to "unlink" the NNID from your current console and then "link" it to your new console. Because the only way to transfer the NNID to another 3DS system is the Data Transfer option in the system settings. But to do this you have to update your firmware.
As far as I know now there's no other method.
And the Gateway emunand uses the same NNID that is linked via the sysnand. Even if both NANDs are unlinked.