QGIS .qgz files are merely zip containers which have the .qgs file embedded, together with potentially some sidecar files.
I have just tried, and on a simple project just zipping the .qgs file alone into a .qgz file generates a usable .qgz project. In recent versions of QGIS, it is missing the project styles database (ending in .db, see https://github.com/qgis/QGIS/commit/c6beb62216fcfbfe3ca64283c09117da53800a98 ) but no errors or warnings appear to be generated, so if you aren't using multiple project style databases functionality, you should be fine.
Assuming this is the case, just therefore use your favourite zip utility to zip up each .qgs file to a .qgz file, say using the command line.