Very coincidentially to us watching Casshern Sins, Ledford licenses the original and Raymond Herrera's summary of it adds a whole lot of sense to the remake:

"Casshan" follows the adventures of Tetsuya Azuma, the son of the scientist responsible for creating Buraiking, an android that turns the robots of Earth against their human creators. Tetsuya transfers his consciousness into an android body to become the robot warrior Casshan, and alongside his robot dog Flender and the wielder of the most robot-lethal gun in the world, Luna Uetsuki, he begins a journey to clear his father’s name, avenge his mother’s death and save the planet from Buraiking.

Why, it's quite like reading the original sci-fi classic Stalker and watching the movie. The legend says that Tarkovsky sent the Strugatsky brothers to redo the script a few times, until they cried in exhasperation: "you do not seem to want any of the sci-fi!" and Tarkovsky replied: "exactly!". They rewrote it one last time without the sci-fi elements and the rest is history.

The point I'm trying to make is that the remake of Casshan is not as removed from the original as Xenoglossia is removed from Idolm@ster. It generally follows the same pattern, but refocuses the whole story. Just to reveal one example, Luna does not need any gun, even an ultimate one. Her blood (nanomachine solution) kills or heals as if she were a god.