Old versions are not available, because it’s strongly recommended to use the latest one. The only reason would be to convert files older than version 3, but this can be performed with the version 3.3.2-light that can still be downloaded.
Version 3.3.3-light is now available on the download page. It can be used to convert old schematics and data displays. Just open them and save them. After this, they can also be opened by the most recent QucsStudio version.