Actually, this movie isn't so much an adaptation of the Mayumura novel as it is a sequel to it, much in the way that "The Girl Who Leapt Through Time" functioned as a sequel to its source material. In both cases, there are new protagonists, with characters from the original books appearing as adult mentors.
The original book in this case is much more a parable about confronting fascism than a sentimental tale of young love, though.
...chief animation director's correction sheets...
I'm imagining every last one of these sheets saying the same thing: "Needs more lens flares!"