TWBX files are zip files. They contain a TWB file along with the data files and images. This requires one additional step once you have the TWB saved.
First, select the TWBX you want to convert. The zip file will unpack in a list on the screen. Find the TWB file in that list and click on it. Then select the Tableau version that you want to convert to. Then click "save as a TWB" and it will download in your download folder.
Once you have the new TWB file downloaded, open the TWBX file with a zip-file utility (for example, 7-Zip). Delete the TWB file that is in the zip file, and add the new TWB file that you just downloaded. You should now be able to open the TWBX file in an older version of Tableau.
Note: I have added functionality for Tableau 10, but it's still in beta. I have tested various conversions, different connection types, and multiple connections. This may change with future releases as new functionality in the XML or other changes may affect the conversion.
I hope you find this information helpful. If you have any questions feel free to email me at Jeff@DataPlusScience.com.
For more tips, tricks, and vizzes by Jeff, visit his Tableau Public page, check out his blog, and connect with him on Twitter @HighVizAbility.