Why is my project file sometimes opened in an older version of Prism instead of the current version?

You can maintain more than one version of Prism on your computer at the same time.

For Windows operating systems, if you double-click on a Prism file name or file icon, that file will be opened by whichever version of Prism you used last. You can manually override that by simply launching the version of Prism you want, then opening the project file using "File.. Open..".