I just recently bought a 3DSXL for myself and have been having a blast with Mii Plaza and Streetpass! My SO wants to borrow my 3DS from time to time to play his own games on the go, and wants to link his Nintendo eShop account onto my 3DS. I have no interest in buying any games, as my cartridges are more than enough for me.
When we tried to link it, the "You cannot link a single Club Nintendo account to multiple Nintendo Network IDs" comes up. I've been reading through Nintendo's forums as much as I can, and from what I understand there really is no way to "log out" of your NNID. The only way is to remove it, and removing it takes your Friends List, Mii Plaza, Streetpass, etc along with it!
Nintendo's FAQ states, "You can format the system to remove the Nintendo Network ID without deleting it. Nintendo Network accounts removed in this way will not be deleted from the Wii U (if linked), and can be relinked to the same Nintendo 3DS system again in the future"
If I format, will my information be retained? Will he be able to buy and play his own 3DS games with his Nintendo Club account? I'd like to note that both of us are first time 3DS users, but he has a Wii U and I do not. He is convinced that I will not lose my important information such as Mii Plaza and Friend List if I format it (because if he format his Wii U, he would still retain HIS information).
I really do not want to lose all the fun progress I've made, is there a safe way to do this so I can keep my data intact?