Great question James! What you can do to accomplish this is:

1. Download the public document

2. Reupload the same document to the client, but as an Internal Document 

3. When you're ready to share the Document with the client again, navigate back to it and click on Share Document. This will send the document to the client again and allow them to access it through the client portal

It isn't currently possible to have a true toggle between internal and external documents, but that is a fantastic feature request. We will add it to our Feedback Center for consideration

Thank you and if you have any other questions please leave an additional comment/post!