Here found a series of leaked images of a prototype 645D:-
It seems that this version of the 645D is a real Evil! It has interchangeable digital back or sensor!
However, the above shown prototype is not the latest version, as the back colour LCD display is just smaller than the prototype that has just been shown at the PIE lately. Let's see if the final production version will have this evil feature or it was just made for convenience as a testing prototype in which the sensor could be changed.