Storing Smart Previews instead of originals

Hi all,

Is there a way to make Lightroom CC store my photos as Smart Previews instead of the full originals? I've started importing photos to CC via Lightroom Classic, and in my classic catalog the photos are stored as originals, but in CC they're stored as Smart Previews. It's working really well as it's saving a lot of space, and the photos I sync with CC don't need to be full size as they're only for social media. The problem is I have older photos in CC that I would like to change from being stored in CC as the original, to just being stored as Smart Previews. I don't need the full original file stored in CC. Is there a way to change the way they're backed up in CC so that they are just Smart Previews?

Thanks in advance!