Default User Experience After Lightning Experience Is Turned On
Where: This change applies to Group, Professional, Enterprise, Performance, Unlimited, and Developer editions.
When Lightning Experience is turned on
All users who exist at this time remain in Salesforce Classic when they’re assigned the Lightning Experience User permission. This is true whether an existing user has the permission at the time that Lightning Experience is enabled or is assigned the permission later on. It doesn’t matter if the user permission comes from a standard profile, a custom profile that has the permission enabled, or a permission set.
After Lightning Experience is turned on
Things change for users who are added to your org after Lightning Experience is enabled. These users get Lightning Experience as their default interface whenever they’re assigned the Lightning Experience User permission—whether at the time they’re first added or at a later date. Again, it doesn’t matter if the user permission comes from a standard profile, a custom profile that has the permission enabled, or a permission set.
All Lightning Experience-enabled users automatically get the Switcher, which allows them to switch between Lightning Experience and Salesforce Classic, as needed.