What will ultimately happen to the upper DU display after the lower DU fails?

When the lower Display Unit (DU) fails on the A320, the system is designed to ensure that critical information remains available to the flight crew. In this case, the upper DU automatically switches back to display the Engine/Warning Display (E/WD) page. This transition occurs because the E/WD is a critical display that provides essential information about the aircraft's systems, engine parameters, and any warnings that may be present. By making the E/WD available on the upper DU, the aircraft prioritizes safety and ensures that the pilots have access to vital information, thereby minimizing the impact of the lower DU failure on situational awareness.