Usernames work like domain names on Cardbox. When your Pro subscription expires, your username is released and other users will be able to claim it after 24 hours. If you renew within 24 hours, your username stays yours. Please note that your personal username link stops working immediately after expiration.
All groups over the free limit of 5 will continue to exist, and all Group Permissions will continue to apply. However, Group Permissions can no longer be set or edited.
All connections you have made over the free limit of 100 will continue to exist. However, you can no longer request to connect to more people.