That's right, Tom. The Show Changes button will only display the latest version of what was modified. There are some other options here:
https://casecomplete.zendesk.com/entries/21908333-Comparing-Project-Versions-and-Baselines
While you could certainly baseline/compare versions of the Show Changes report, for what you're looking to do, I think that publishing (securely!) to Requirements.cc is probably the best option. Each time you upload your project, a history is kept of exactly what changed. You can view old versions of your project, and even see the differences between versions. The interface looks a little different than what's in this article, but the functionality's still the same:
http://blog.requirements.cc/post/View-and-Compare-Versions.aspx
Jason