By default, the ability to switch to the classic UI via the admin consoles has been disabled. Therefore the pop-up icons that was seen when mousing over certain console icons, allowing access to classic UI, are no longer displayed.
Every console that has a Classic UI version in /libs/cq/core/content/nav can be re-enabled individually so that the Classic UI option once again pops up over the console icon when it is moused over.
In this example, we are re-enabling the Classic UI for the Sites console.
Using CRXDE Lite, find the node corresponding the the admin console for which you want to re-enable Classic UI. They are found under: