Hi Audrey,
If I understand your request correctly, there's really not an automatic versioning function in CaseComplete. However, you could manually increment your use case versions using the notes section, or possibly a custom field that you could maintain.
In addition, we've also got a feature called Requirements.cc that allows you to upload and view your projects in the cloud. The feature has a view/compare function that might match your needs. It's still in beta right now, but we're planning on releasing it to everyone very soon. You can learn more about it and find instructions on how to try it out in the blog below. Start reading from the bottom up, and the description on the view/compare function is the second post.