Are savefiles kept when I download / install new updates? I enjoyed the story but I don't want to go through everything again just to see the updated content
Don't worry, your savefiles are kept when you delete the old update and open the new one.
Save files are kept in 2 locations on desktop platforms so they won't be lost if you delete an old version of Sileo. On Android, make sure you install the new apk over the old one, uninstalling will clear your save data.