Not much about Nintendo’s online service has been revealed when it comes to the Nintendo Switch, but it’s clear that Nintendo will be phasing out the Nintendo Network ID (NNID) in-place of Nintendo Account.
You would have created a Nintendo Account if you played Super Mario Run or Fire Emblem.
As of right now, you can login using your email address and claim your username on your profile page. You can do that HERE.
It looks like once you’ve locked it in, it can’t be change so definitely choose wisely.