http://bbs.saraba1st.com/2b/forum.php?mod=viewthread&tid=1110639我这个贴的某楼有写
首先无论怎样都不能选删除NNID，删了NNID就什么都没有了，真正意义的啥都没了。
接着
If you do not want to permanently delete the Nintendo Network ID, you can choose to "Delete User" instead. Nintendo Network accounts removed in this way will not be deleted from the Nintendo 3DS (if linked), and can be relinked to the same Wii U console in the future
What to Do:
On the Wii U Menu, select the Mii character in the upper-left corner to access the "User Settings."
Tap "Delete This User."
If this selection is grey, there is only one account on the system, and it cannot be deleted. You will need to format the system instead. How to
点左上角的mii，选择删除user，如果只有一个user，那就只能format机器。
我的理解是这样之后你就可以新建一个user，然后可以重新绑nnid，从上面的解释可以肯定新user能绑回原来的nnid（原来的nnid绑死在这个机器里），但是不知道能不能新绑一个新的nnid？
不过一旦删除user或者format机器，所有save data都会消失，所以无论能不能换nnid，这步都是不可避免的
我也建议等等吧，我甚至都懒得去试这个傻逼系统了，盐田聪说会更新成正常的帐号系统的，现在就忍忍了。