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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...
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Source: https://writers.stackexchange.com/a/30280 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/30280 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
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.