I would say that none of the exterior parts are interchangable, but I'm pretty sure a good 90% of the interior is the same. The MKZ/Zephyr's interior is titally different that the Milan and the Fusion.
Everything is interchangeable. The only exterior parts that are different is the fascia, lights, fenders and I think the hood, but if you really wanted you could swap them all for Fusion or even Zephyr/MKZ parts. Same in the rear. The interior is 100% interchangeable and the suspension and drivetrain is exactly the same, so no need to interchange anything.
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