@OldManGeorge just wanna say thank you very much for the guide. Thanks to it, I was finally able to switch over to OneNote from Evernote after at 3 previous attempts over the years failed. I was seriously worried I was gonna be stuck paying $75/year for Evernote just because I couldn't find a way to properly move my data between services.
That said, there are some peculiarities on the OneNote side I'll point out:
1st of all, on Windows 10, you'll want to use the OneNote for Windows 10 app and not the OneNote 2019 app. The former uses the paradigm I describe below, while the latter mixes that paradigm with Evernote's, with very confusing results. You'll also want to have the OneDrive application running. Microsoft doesn't mention it explicitly, but it's what OneNote for Windows 10 uses as its sync backend. If it's not running and/or you're not signed into it, OneNote for Windows 10 won't sync properly. This took me a few hours to figure out; I believe the vast majority of OneNote sync problems reported are due to it.
OneNote for Windows 10's sync and file paradigm is different from Evernote's. While Evernote does operations locally and periodically syncs them, OneNote for Windows 10 is basically a desktop client that reads and writes files in real time from cloud filesystem. If the client is unable to write to the cloud filesystem, the operations queue up until it can.
Following from the above, OneDrive (NOT OneNote web) is the canonical location and record of OneNote files, not whatever you see in the client. To get an imported Evernote notebook to show up in your client, find it in OneDrive, where it will be in your Documents folder with the same name as the imported file. Right-click it and then click Open in OneNote, then follow your browser's prompts to open it in OneNote for Windows 10. If you want to delete a OneNote notebook, close it in the client (right-click -> Close this notebook), and then delete the corresponding file in OneDrive.
Confusingly, OneNote web is missing some of OneNote for Windows 10's features. I don't recommend it as a client.
OneNote shows recent notebooks in the in-app dialog for opening notebooks, even if the recent notebook as been deleted or you no longer have access to it. It's nice for history, but it's quite confusing.
The TL,DR of this is: if you want to do work within a OneNote notebook, open the notebook (if it isn't already open) in the OneNote for Windows 10 app and work on it there. If you want to do perform a file operation on a OneNote notebook, such as renaming or deleting it, close it in the Windows 10 app, and then rename (and then reopen) or delete it from the OneDrive Documents folder.
As to why I'm switching after over a decade with Evernote: price (my entire Office365 subscription costs less than my Evernote one) and much better freeform support. In OneNote I can, for example, hand draw a graph and then label the axes in text boxes, while Evernote forces writing and typed text to be separate. Also, the Evernote for Android suffers from awful pen-input latency, while the OneNote for Android app doesn't.