How to Reset the Named People Album in Photos on High Sierra and iOS 11
Starting with iOS 11 and macOS 10.13 High Sierra, iCloud Photo Library will sync the named faces between all the Photos Libraries on all devices. If the faces have previously been named inconsistently across devices, it may be easier to remove all named faces from the synced libraries and start over with an empty People album.
On a Mac with macOS 10.13 High Sierra you can reset the faces database:
- Click People in the sidebar.
- Select all people at once in the People album by entering the key combination ⌘A.
- ctrl-click the selection and then select "remove all". This should prompt you, if you want to reset the faces.
https://support.apple.com/guide/photos/view-photos-by-whos-in-them-phtad9d981ab/ mac
If you are using iCloud Photo Library, do it on all devices, before you start naming the faces again to avoid wrong faces names syncing back from other devices.
To save most of your previous work with naming people, I would assign keywords with the names to all photos that have been named correctly. For each People Album, where you want to save the named faces, select all photos, open the Info panel with ⌘I, then enter the name as a keyword in the keyword section. I would use the underscore instead of space character when typing name keywords.
This way you will later be able to find quickly all photos of a previously named person by searching for the keyword.