Arnd LayerToday I learned that you can’t just replace a photo in Apple Photos (on Mac) with another file, ripping all the references to the photo alive.
But there is a workaround. If you have a photo editor - in my case Pixelmator Pro - that can edit photos from your library, you can import the new version as a new layer and delete the old one. Save it back to Photos and you’ve got what you want.