My guess is they would wait until at least MY07 to complete at least three model years with the existing design, and then Bangle-ize the X3 like the rest of the line, including the new engine. It just seems to make sense from a marketing perspective. I'm not in the car design and marketing business, but I don't think it would make sense to go to the expense of fitting a new engine into the existing X3 chasis, only to turn around a year or two later and redesign the whole vehicle.
I personally would like to see a couple years of the new engine in the mainstream marketplace before I bought one anyway. There is a lot of new technology that needs to be proven in the general car-buying public.