How does Photos handle Aperture versions?

jfaughnan wrote:

but there was only one JPG in the converted Photo Library. It's as though Photos was using the Aperture transform directions to render the image -- but it could not then be edited.

Photos is handling the Aperture edits like edits done in an external editor. Both together are appearing in Photos as only one photo. The edited version will be used as the current version of the photo with the Arture original as the original.

To save the Aperture edits in a high quality, it is essential to create high quality previews in Aperture before you migrate the Aperture Library to Photos. When I want to edit a photo in Photos, that has Aperture edits applied, that I'd like to keep, I duplicate the photo in Photos ad revert the copy to the original. Then I can edit a second version.

I kept an older Mac on Mojave to be able to fix older Aperture libraries. Without an older Mac you could use Retroactive to be able to create better previews, if you want to be able to run a patched, limited version of Aperture to repeat the migration on a system version, that no longer supports Aperture.

This might help:

https://www.takecontrolbooks.com/resources/0176/site/chap05.html

Note, however, as you are discovering:

If your images in Aperture have been modified via adjustments or brushed-on edits, the edited versions and the originals are imported together into Photos.

Unfortunately, you can’t modify these Aperture adjustments and edits in Photos. If you want to start from scratch, click Clear Adjustment or Revert to Original in the Edit pane to start fresh, or choose Image > Revert to Original.