After all the fretting I did, it turns out that I only had two duplicate notebooks between OneNote for Windows 10 and OneNote 365. Of course, one of them is part of the series bible for the Paladin League, so a majorly important notebook.
So far, the Paladin League notebook has had the exact same pages in both notebooks, so I've just needed to delete the page and check the next one. It's tedious and I'm doing a page or two now and then.
The big issue is the other notebook that's duplicated. It's for a series I'm thinking about writing, but haven't done anything with, but I have the names in for the hero and heroine. Different names in each notebook for the same characters.
I have no memory of what happened, but my guess is that I picked names and the characters shot back at me and said, "That's not my name."
If you think I'm joking, you'd be wrong. Out of all the books I've written, all the ideas I've had, my characters have literally allowed me to choose a name once. For one heroine.
All the others? They tell me their name. Or they make me guess and refuse to work until I get it right.
For example, I have an idea for a series that spins off of the Paladin League. One of the heroes in that book told me what his name was. I didn't like the name and told him that. He said to me, "Let's just use the name as a placeholder and I'll pick something else later."
He lied to me. He never picked another name and by the time I was making notes and realized that, it was too late. That name was stuck.
But I digressed. Sorry. OneNote. I managed to just port over all the other notebooks to OneNote 365, so it ended up being easy. I even ported over the two duplicates and marked them with an emoji to keep them separate. So this fiasco isn't as bad as I feared.
The other series has a duplicate with almost no information in it, and it should be quick and easy to jettison. The Paladin League is going to take more time to go through because both the duplicate and the original are loaded with pages. I don't know how I managed this, but I'm blaming Microsoft.