In addition to this, they’ve also confirmed that system software 6.0.0 will release alongside Nintendo Online. Obviously, you’ll need to update in order to the Nintendo Switch eShop as well as Nintendo Switch Online.
Interestingly enough, you won’t be able to change the Nintendo Account that is linked to your Switch after updating to 6.0.0
Here’s what Nintendo had to say about that:
After installing the system update planned for September 18th, 2018 (Tuesday) at 5pm Pacific Time, it will no longer be possible to unlink your Nintendo Account from your Nintendo Switch user.
Prior to this change, it is important to confirm that your Nintendo Account or your child’s Nintendo Account is linked to the correct user.
Nintendo Switch system update version 6.0.0 has been scheduled for September 18th, 2018 (Tuesday) at 5pm Pacific Time. After this system update is released, Nintendo eShop will be under maintenance for a brief period of time. Nintendo eShop will not be accessible during the maintenance window below. We apologize for the inconvenience.
After the maintenance is complete, Nintendo Switch Online memberships will be available for purchase. Nintendo Switch owners won’t automatically become members when the service starts, so they will need to purchase an Individual or Family Membership in order to avoid disruption of their online play in compatible games. Memberships can be purchased or redeemed through Nintendo eShop and Nintendo.com once the maintenance completes.