Yes, no, maybe. Generally no, not necessarily. A new SoundHelix release might very well expect a different XML structure than the previous version, so you should not expect your existing XML files to work right away. Currently there are no plans for providing transformation scripts which upgrade an older XML file to a newer one. In most cases, only slight modifications are required. You should look at the change log to find out what has been changed between SoundHelix releases.
Note, however that older XML files might still work after a structural change without any errors shown, but some features might not work as expected. This is because there currently is no proper XML validation in place. Also, song names that produced a certain song with a certain SoundHelix version might produce a different result with another version, even if the XML structure has not changed.