Lightroom CC and Lightroom Classic Integration Question

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Simply put, how can I, at any given point, take a photograph whose full size raw file is uploaded to the cloud already and replace that full size raw file on the cloud with just the smart preview of the photo? The desktop Lightroom CC app, if it knows where the locally stored raw file is, will automatically upload the full res raw back to the cloud again...can I somehow prevent this? Is there a clever workaround? For context, I have only 20GB of storage and I want to control which photos in my entire catalog have full res versions stored on the cloud for when I want to edit them in the near future, and which just have smart previews uploaded. Once my work with the full res raw files on the cloud is done, I'd like to clean that space up and replace the raw file with just the smart preview. Is this possible? 

[Jim Wilde]

If you've only got 20GB of cloud space, then you must have the Photography Plan which includes Lr Classic, yes? In which case, the typical workflow to manage within that space if importing images into one of the Lightroom apps (which always upload the full res original to the cloud) is to sync the Classic catalog with the cloud. That way, any original added to the cloud will also download into the Classic catalog. Once it's been received into Classic, the image is then "unsynced" from within Classic (by the simple expedient of removing it from the All Synced Photographs special collection in Classic), which in turn causes the image to be deleted from the cloud. Once deleted from the cloud, the image is then synced again from within Classic, which now only uploads a Smart Preview of that original to the cloud, and which does not count against the 20GB space allowance.