Kameleon,
Yep, Gino's right. The major differences between the AH-64A and D is the fuselage avionics sponsons. The D model ones are considerably larger and they are not shaped identically as the ones on the A model are. The sponsons were enlarged to accomodate the added avionics systems and radar-associated gear.
Converting an A to a D is possible, but tricky. There are no aftermarket sets available to do so, so you've got a lot of scratchbuilding to do. I'm doing it right now with my Revell AH-64 in 1/32. It has taken quite a while to do, but I'm happy with the results.
In addition to the avionics sponsons, the position lights have been moved from the wingtips to the engine nacelles; low-airspeed sensor arms have been added to the nacelles as well; and the cockpit interior is completely different.
Hasegawa has recently released an AH-64D kit in 1/48, and from what I know, it is the best Apache kit available.
Hope that helps!