I just received the "official" DCC-EX motor shield named "EX-MotorShield8874". I ordered it directly from the DCC-EX web store.
My main reason to replace the Arduino shield with the DCC-EX one, is that this DCC-EX shield makes it easy to have only one power source with all the components: motor shield and Arduino Mega. Space is limited in the test bench ...
I unplugged the Arduino motor shield and positioned the new one. I plugged the power source and it seems to work immediately!
I still need to test it with an actual locomotive, but it would easy to reconfigure the Arduino with the DCC-EX installer or rather the Arduino IDE, since I modified the source code for the display.