Although the Nintendo Switch does not officially launch until March 3, it is now possible for you to pick the name for your online account. By heading over to the Account Settings page over at the Nintendo Account website, you can select a User ID that will work on the new hardware.
This User ID differs from your nickname and previous account names at the Nintendo Switch uses a brand new system for its accounts. While linked to your overall Nintendo Account, the User ID is a separate system that replaces the NNID used by both the Wii U and Nintendo 3DS as the company tries to have a more unified online service. It also means that any purchases made on the eShop will be tied to your overall account, making it easier to manage any downloadable games across all Nintendo consoles.