According to comments made by Furukawa during Nintendo's recent shareholder Q&A, the plan is to utilise the existing Nintendo Account system to make the jump to the next generation as smooth as possible for customers. Here's part of what Nintendo's president had to say, courtesy of a translation by Twitter user Genki:

Shuntaro Furukawa: "As for the transition from Nintendo Switch to the next generation machine, we want to do as much as possible in order to smoothly transition our customers, while utilizing the Nintendo Account."