The first is that we have the introduction of another monster, and together with it comes the idea that this movie focuses on a battle between these creatures. That is reinforced by the tagline.
The second difference is that we know have an helicopter entering into the battle. I imagine that the idea here is to make the movie seem more action packed.
The third difference is the change in the title’s design. As a matter of fact the title has been also changed from D-War to the more on the nose Dragon Wars. And the fourth difference is that the colors are a little brighter in the American version of the poster.
Honestly, I dislike these changes. I think they mostly take away from the simple appeal of the giant snake attacking Los Angeles. And much like it happened with the American version of the poster for The Host, they make the movie seem cheesier. Too bad, because I really liked the Korean version.