Currently it is possible to switch between the old and new Outlook versions.
To do this, have Outlook open and click the switch button located at the top right of the window beneath the close and minimize button.
After clicking on the switch, Outlook may prompt for feedback. Select to skip feedback and Outlook will close and begin opening the opposite version.
This switch can be seen in both versions of Outlook and can be used to swap between the newer and older versions.