After you have configured a Dynamics 365 Portal, the option to change the Portal Audience is disabled on the Portal Details page.
This option cannot be changed on the Portal Details page.
You can change to a different portal audience and type by using the Manage Dynamics 365 Instance option:
Go to the Dynamics 365 Administration Center, and then select the Applications tab.
Select the application row for your Portal Add-On, and then select Manage.
Click Manage Dynamics 365 Instance.
Click Update Dynamics 365 instance.
Use the options that appear to change the Portal Audience or portal type.