If you choose not to upgrade the group or you don't see an option to do so, the group will eventually be archived. Before this happens, members will see an announcement when they visit the group. If they choose to continue their membership, they can keep using the group after it's archived.
Keep in mind that the new groups format was designed to help you share with the small groups of people in your life. If you’ve been using your old group to promote your business, we recommend you create a Page instead. Learn more about thedifferences between groups and Pages.