Download pre-existing photos to iCloud for Windows

See the instructions in: Set up and use iCloud Photos on your Windows PC - Apple Support

You should not need to do anything.

If you logged in to the iCloud for Windows app with the same Apple ID you used for iCloud on the iPhone and iPad, the photos should automatically appear in the iCloud Photos folder in Windows Explorer (assuming you are using iCloud Photo Library on your iPad and iPhone).

My understanding is that only new photos that are taken after iCloud for Windows is set up will download automatically. That is based on the following statement from my previous link:

New photos and videos you add to iCloud Photos will automatically download to your PC.

There are instructions in that link for downloading prior photos.

OR -- I'm wrong and completely misunderstanding it!

Yup. Key words: "automatically download". The iCloud Photos Folder will still show other photos taken before iCloud for windows was set up, they just won't be automatically downloaded to the PC.

I never said they would be downloaded. Just that they'd be there.