Not sure what you mean by "skeletons", but if you have some of your 20GB allowance already used up, the logical assumption is that you have some originals stored there (such as photos imported from LrMobile on a mobile device, or imported directly into the Lightroom Desktop or Web apps). Any of those images will already be in the LrC catalog (assuming sync was fully up to date between LrC and the cloud), but if you want to identify and remove those originals from the cloud (and replace with Smart Previews) the easiest was would be to use the Lightroom Desktop app:
1. Open LrD, click on All Photos to populate the grid with all the cloud images.
2. Create a (temporary) new Album, give it a unique name.
3. Click on the Filter icon (at the end of the top centre search bar), and select Sync Status from the filter bar.
4. That will show various entities, such as Synced from Lightroom Classic, but the one you are interested in will be "Synced and Backed Up", which is all the images stored in original form.
5. Click on that "Synced and Backed Up" entry and that will populate the grid only with those images.
6. Cmd+A to select them all, then add them to the new temporary album.
7. Wait until the equivalent collection appears in LrC.
8. When the collection has appeared, unsync it by clicking on the sync icon on front of the collection name. That will have no affect on the synced images in that collection, they will remain synced.
9. Select that collection, then Cmd+A to select all the contained images. Then, keeping them all selected, click on the All Synced Photographs special collection, then right-click on any one of the selected images and "Remove from All SYnced Photographs". This will remove all those images from the ASP and will also cause them all to be deleted from the cloud.
10. Monitor the LrD app until all those originals have been deleted from the cloud. When that's done, and assuming that you want them all back in the cloud as Smart Previews, go back to LrC and re-enable sync for that collection, which will upload them all as smart previews. Any that you don't want to resysnc, remove them from the collection before enabling it to sync.
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