As we roll out the new version of Express, we obviously intend for files created in the previous version to be usable there as well. For some users (specifically, those we identified as never having used the mobile app), migration of the files should have happened automatically. For the remainder, files will have to be migrated manually. In either case,files should not be lost.
However, if you find your file missing in both locations, please log an issue using the Adobe Virtual Assistant at https://helpx.adobe.com/support.html. When you hit the link above, the team has direct access to your account and information and can help resolve.