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Not being a transmission specialist, I can't see any reason why you can't physically do it (may need a different drive shaft). The software/computer stuff is what I think you would need to investigate. I don't know if the brains of the transmission are housed in the tranny, in a different module, or in the Durango engine computer. If they are in the engine computer, that could be a problem where you would also need a computer from a newer D. In that case, the costs would add up quickly.
I assume you want the performance and gas mileage advantages of the 8-speed. If that is the case, the least expensive and most reliable way to do it is just buy one when they come out. Start saving now.