They will grow back, some faster than others. All my tanks have cories on round store bought gravel and do fine. I ran into a problem once when I added root tabs for the plants. If they appear red at all I would add some antibiotic. I believe if they've actually managed to scrape the skin on their mouths it can be an entry for the bacteria and can lead to septicemia. Increase your wipe downs and water changes to speed healing along.