The 2019 Wave 2 update has had some changes to the “Enable only the Unified Interface” setting.
Paul’s blog describes how to enable the Unified Interface by default.
Now with the Wave 2 release; “Enable only the Unified Interface” has been removed from the “System Settings”.
This has been moved the Power Platform Admin Center and renamed “Use Unified Interface only”.
How do you change this setting now?
Navigate to https://admin.powerplatform.microsoft.com
For the Dynamics 365 environment you want to change, select the option “Settings”
Now you can toggle the “Use Unified Interface only”