My main archive is still in Aperture (running El Capitan on a pre-retina iMac 27″!)
I have been meaning to write something describing the options people have. Until then, here is a brief look at them.
1) Move the Aperture library to Photos. The adjustment data will be discarded, but it will generate full-size previews, so your images will appear correct. Any subsequent edits will be made to the preview, or you have to Revert to Original. Note that Photos does have a Duplicate command, so it should be possible to duplicate, revert the duplicated image and edit the duplicate image without disturbing the Aperture edit. Most metadata will be moved, but not all (e.g, some custom metadata isn’t moved, ratings become keywords and some other odd transformations I can’t recall at the moment). While older versions of Photos did a much better job of preserving Aperture data, later versions take the migrated data and migrate it again, discarding the relevant Aperture data (that is my understanding, I am not 100% sure).
2) Use the LR importer. This will bring some of the metadata (I believe ratings and keywords are preserved properly). Edits are not preserved at all, so you would need to export your edited images (you can search for images that have adjustments, so you don’t have export every image) and import them into LR. This is quite clunky, of course. You could stack the exported images with the originals, to approximate what Photos is doing.
3) Use Capture One’s importer which I have no experience with, but it tries to import some of the Aperture adjustments.
When you export images in Aperture, the metadata is stored in the exported file. Aperture can also export an XMP sidecar.
RAW Power 3.0 will be a front-end for the Photos library, but there are a lot of limitations imposed by Apple. For example:
1) There is no ability to read or write keywords, or any metadata except “favorites” and creation date
2) Aperture adjustment data is not readable, because it will have been converted before RAW Power could read it.
I plan to add ratings and flags to the Photos library, though it’s crazy what I have to do to get it to work.
Some restrictions may be relaxed in the future (like keywords). I am pretty sure the adjustment data is a lost cause once it is in Photos, though.
Re: a conversion program. I appreciate your desire for such a thing. There are a few options, which are not great:
1) Convert Aperture adjustments that also exist in Photos so that things are non-destructive in Photos. This is a very small set. Even Exposure doesn’t technically exist in Photos (it’s part of the Light adjustment, which isn’t the same thing)
2) Convert Aperture adjustments into RAW Power adjustments. This is a larger set, but Aperture has way more adjustments than RAW Power, including brushing which I’m still working on.
I sometimes think about writing #2, but I’m so buried with work, it’s hard to imagine how to carve out the time necessary to do a good job with it.