I've tried searching for this, but couldn't find any info about it, so sorry if this has already been posted somewhere.
I know NNIDs are being phased out in favor of Nintendo Accounts, but at the same time I think an actual ID system will make a return (e.g. for friend registration). Reason being, on the Nintendo Account settings page, there's an option I've not seen before: 'Change User ID'. When you click on it, it opens a menu which is identical to the password change option, both aesthetically and functionally, even though the password change option is a different option. Changing the email and nickname are still different options so it's not them either.
This pretty much suggests that its an incomplete feature, so probably the Switch's account system?
ift.tt/2kELMoa (F12 was used to remove name/email, I changed nothing else)
But because: a) NNIDs couldn't be changed, and I doubt they would plan on changing that this far into the Wii U/3DS lifecycle b) IDs being referred to as 'User IDs' rather than Nintendo Network IDs c) It evidently being an incomplete feature, and no explicit references elsewhere I think it's a strong indication of the new ID system.
Direct link: ift.tt/2koIdE0
Discussion started at here by switchvoid