Does anyone with experience have suggestions on how best to go back, edit, and rearrange scenes and details? You know, without me dying in the process? The whole book needs to be restructured.
Yes, this is going to be a death march. But the thing that is going to kill you is not this rewrite, but the next one and the one after that as your story gradually become more and more disorganized through the process of revision if you fall into the trap of trying to get there by rearranging scenes.
This would be a really good time to get a copy of Robert McKee's Story and pay particular attention to what he has to say on the peril of trying to hold on to all of the "good" scenes in your existing story. A story is not a collection of scenes. It has a very definite shape which you have to find, and any scene that does not fit on the arc of the story shape has no place in your story no matter how brilliant it may be individually.
What Lauren says about outlining may well be the best technique for designing and capturing the story arc. Be aware, though, that a simply listing of events does not necessarily capture the arc of the story, which is essentially a moral progress to which individual events are tangential. For evidence of this, look at the plot summaries you find of so many books and movies on Wikipedia. Many of them sound like a completely shapeless series of events because they don't capture the moral mainspring of the story. If it is going to guide you usefully, an outline has to capture the moral arc of the story.
Also beware the trap of treating outlining as a way to fit scenes in. Look at it that way and you will fall into the trap that McKee describes.
To survive your trip through the valley of death, therefore, find the moral arc of your story. Keep anything that belongs to that moral arc. Trash anything, however brilliant, that does not. You may still have to pass through the valley of death a few times, but you will at least have a chance of getting out alive.
Sorry, you may have to die a bit.
Make a copy of your plot outline. (You have an outline, right? No? Then you'll have to create one after the fact. Read through your existing book and pick out the outline from what you wrote. That's your "original.")
Take your original outline and put it next to the copy. On your copy, start re-outlining with your new ending in mind — as if the original didn't exist. Use your original as a reference to say "A goes here, B goes over there, C is out, this is a new C which I'm going to call Ralph."
When your new outline is as good as you can make it, hand it off to someone to see if it holds water.
When it does, make a copy of your manuscript the same way you did with your outline. Open the copy and start reading it along with the new outline. When you hit parts of the new outline which have changed from the original, hit the enter key a few times to create space, and then start writing the new material. Add and remove pieces as needed.
There is no shortcut, I'm afraid. Honor the better ending and dive in.
The whole book needs to be restructured.
At least the way I write (more of a discovery writer), a new book would have to be written with the ending in mind. I would keep the old book for reference (I presume I have infinite disk space and never throw anything away anyway); mostly for some hard-crafted descriptions, word choices and even dialogue.
But I would start from the beginning. No matter how one writes, scenes grow from previous circumstances and should feel mostly inevitable. They shouldn't contain much of anything that doesn't contribute to the story. If I really could just go back and rewrite 25% of a story and get a completely different ending, that means 75% of what I wrote had no impact on the story, and to me that is unacceptable writing.
Start over. The old book is a bin of parts you might be able to use or adapt. By starting over you don't have to re-use old characters and scenes, you can invent new ones purpose-fit. Brainstorm about the main characters, then the supporting roles. If you are compelled to change the ending, you must have realized something, at least subconsciously: Figure out what your story was really about, in more concrete terms, and start over.