Yeah I did see that and tried it, but it didn't work. Once someone upgrades, can the upgrade be undone and then redone? Just trying to think outside the box of a way to fix it.
After upgrading, all my quizzes are gone and I cannot edit a new quiz
The only way to undo an upgrade is to restore the site from a backup.
But before you do that, what happens when you regrade the quiz attempts?