In fact the Due can never be 100% compatible, even if they can map pins to the correct place (unlikely but maybe) and boost them to 5v (easy with digital IO but unlikely they will, probably impossible for analogue IO without a lot of external HW) there is the current-handling ability of the ARM pins, most are only a few mA and nowhere near the 20+mA we expect from an ATmega.
I'm dying to see how they handle this. The photo shows a Mega look-alike so maybe they have got a workable pin mapping, but the other issues may cause a fork in the product line.
Rob