Go to Settings - Cloud Storage & Sync and check 'Only Download Smart Previews'. That should automatically replace the originals you shot with the phone with smart previews once they have been uploaded to the cloud. Don't be fooled by the small 'RAW' or 'DNG' icons you see in the upper right corner of the images. That doesn't mean that the iPhone has a local copy of the raw original, but only that the image is from a raw original. You will even see these icons on images that were synced up from Lightroom Classic, which are always smart previews.