usually if there's an sd card slot on the screen, then there's a seperate cable for the card reader that goes to the motherboard.
The board will automatically default to it's own card reader - I presume there is an option in the firmware setup to choose the screen reader.

Contact duet - not only are they a very helpful company - they'll know exactly how to change the default card reader.

Fascinating looking machine, new one on me - how easy is it to mess with the colours ?