I've begun early experiments in converting from Aperture to Apple photos. I've not found any good user descriptions of the process. This post will be updated as I do experiments or use the results of experiments to refine my searches.
For my first trial I created a new project from a single JPG image and one version using Aperture under Mojave. I copied it to an Intel Air running Monterey 12.1 and opened it in Photos.
Photos said it could not open the Aperture library. It did not say why.
I then regenerated full sized previews per Apple's directions and I tried again. This time the import proceeded. I could see the Version of my original photo but not the original. I then chose Revert to Original and Photos displayed the original image. Then I tried "Undo Revert" but that did nothing. My Version was lost.
Opening the package I could see a file in the originals folder named with a GUID and a .jpeg extension. It had a size of 2.3MB which is the size of the Aperture original.
I tried to repeat the process but Photos declared that "the content of the Aperture library ... has already been migrated to Photos". The famous léonie of Apple forums explained - the conversion process changes the extension to "migratedphotolibrary", I renamed the extension to .photolibrary and I was able to repeat the import process.
On repeating I saw my Version image. I then tried to edit it and got this message:
"Cannot Start Editing Photos cannot edit this image because it uses an unsupported format"
I explored the new Photos library (Package Contents) and could not find any JPG other than my original. I am puzzled as to how Photos generated the version I could see.
There does not appear to be any way to see the original image other than by reverting to original -- which cannot be undone. There also doesn't seen to be any indication that an original exists! (This appears to be a global Photos problem however).
If one wanted to see both a Version and the Original one would need to export from the Version rendering then open that in Preview, revert to Original, then compare.
I finally resolved the stacks Aperture created when I migrated from iPhotos (original image and iPhoto edited). The trick was to find all photos with keyword iPhoto Externally Edited (or similar) then save them to an album then in that album remove any that weren't stacked then unstack, delete externally edited, and re-edit originals (non-destructive).
Then I did "Reprocess Originals" for about 8000 raw images; about 6,000 were reprocessed and about 2,000 didn't need it. That leads Aperture to slowly regenerate previous/thumbnails. It's been about 24h and that is mostly done, I find that I have to periodically pause the task then restart Aperture (or my laptop) and resume. If I really need to regenerate all full sized previews that processing may take a week or so.