https://www.polygon.com/2018/9/13/17855056/nintendo-switch-online-launch-user-accounts-unlink-how-to

A Nintendo Switch system update on Sept. 18, coinciding with the launch of Nintendo Switch Online, will lock Switch user profiles’ to their linked Nintendo Accounts, the company said in an email to owners this morning.

That means users should, in the next five days, make sure that all Switch profiles in their home (theirs and any kids’, for example) are assigned to the appropriate Nintendo Account. It will be impossible to unlink these Switch user profiles from that Nintendo Account later.