@Roboter Hersteller writes "What Cortex materials will be backward compatible for the V5 system? Will you be able to use the sensors and 393 motors or will will all electronics need to be upgraded?"
@VEX Support "Technically you will be able to use the legacy sensors, 393 Motor, Motor Controller 29 & Power Expander.
However please check the Game Manual to verify if they are competition legal."
So this answer is correct, but it needs to say "legacy sensors" and "a Power expanded driving a Motor controller 29 and Motor 393". The power rail on the 3 pin ports are not enough to drive the 393 motors but the combo of the Power Expander, battery and the other stuff is. So far the motor expander and motors have not been mentioned.
Just wanted to be clear based on what we've seen and has been written by VEX says this in other places.