The angular velocity profile of a double-cardan joint is perfectly symmetrical on the input and output sides, meaning the axles won't chatter when the wheels are turned. The only other axle joint that achieves this is the Rzeppa joint, but those are extremely rare in RC. Also they allow the axles to articulate more than single-cardan joints (commonly referred to as CVDs, though not actually constant-velocity as the acronym suggests), so the vehicle can steer more sharply. With DCDs installed you no longer need the steering-limiter screws installed in the front hubs.
The pieces from the standard CVDs won't fit. However, they only offer a meaningful benefit in the front, so there wouldn't be any point in replacing the rear CVDs. That means you'd only need to buy 2 instead of 4.
I have them on all of my XV-01s, road and rally, and they work great. I built one with CVDs in the front to save money and ultimately decided to switch it to DCDs like the rest. The tiny touring-car-size wheels and tires don't have enough inertia to damp the vibrations caused by the CVDs in sharp corners, unlike a monster truck for example, so the difference is definitely noticeable.