The Nintendo Switch doesn't launch until March 3, but you can claim your Nintendo Account User ID for the console now.
If you're planning to pick up the Nintendo Switch when it launches on March 3 - yes, it's just 10 days away - you can go ahead and claim your username right now. There's been a "User ID" field added to your account, and that will be your Switch name. This is a separate name from your Nintendo Network ID, but both accounts will be linked.
If that sounds a little convoluted, well, that's because it is. At this point, you'll have three different Nintendo IDs. There's your Nintendo Network ID, your Nintendo Account, and your Switch User ID. It's possible that the Nintendo Account may eventually replace the NNID, as we learned last week that all eShop purchases will now be tied to your Nintendo Account, and not to the console. I could see the NNID being used for legacy devices only.
Regardless of how Nintendo decides to handle its handful of accounts going forward, if you want to get your preferred username for Switch, you need to register it now. All you need to do is head over to Nintendo's site and log into your Nintendo Account. Once that's done, go to Account Settings and fill in the "User ID" field. Then you can just sit back until Switch launches March 3.