Does Google Photos replace images on the device with low quality versions the way iCloud does? On iCloud, under “optimize storage” there is an option to save photos to iCloud and replace the ones on the DEVICE with low-quality copies.
Does Google photos work the same way? Does selecting “high quality” or “free up space” replace original photos?
Uploading photos to Google Photos doesn't replace or change anything on the phone.  The application will ask you if you want to remove the photos from the Camera Roll after uploading them.