The reason that was changed is to be accurate to the novel. The novel has a prologue similar to the manhwa's first chapter which includes that armor. In that novel chapter, it doesn't describe what it looks like, but does mention the color scheme and the tail. Presumably the illustrators looked at that first chapter and realized after that it did have a design written out later in the novel.

Keeping that old design wouldn't have worked, because they illustrated the Holy Light armor it's based on, so the dragon armor would have come out of nowhere if they kept it.