This question is open to individual reader opinion, so a key component of this answer is your defense of your answer. I also believe that at the core of this question is the idea that the changes being requested are somewhat major changes to the story. I don't believe that the spirit of the question is asking about changes to the story that are along the lines of simple name changes.
I will admit that I have thought about a specific change to the story that I've always been curious about why Schmidt didn't implement. If I had to implement a change to this novel, I would seriously consider changing the setting. I don't feel the need to change the location of the setting, but I do think that changing the time period is worth serious consideration. I would set the story in modern times versus the Vietnam era. I don't think that placing the story in modern times would have changed that much of the story's themes either. The conflicts in the Middle East have committed plenty of soldiers from the United States, so I think that Mrs. Baker's husband could still be a soldier. That source of internal conflict for her could still be present if the story was set in the present day. Holling's sister could still be just as angst-filled as ever too.
My reason for changing the story to modern times has nothing to do with character or plot. I would change the story to modern times for reader purposes. I think a modern setting would make Holling, his family, and the Bakers' war situation more relatable to present-day readers. Schmidt's setting is great. It is unique, and that makes it stand out against the plethora of other young adult novels that exist today, but standing out isn't necessarily a good thing. It's possible that setting the book in modern times would have made the book that much better because it resonated with more readers, and that is why I would change the setting and little to nothing else.