I think I actually found the solution: if I backup and restore a course with users, the gradebook seems to be intact. I delete the inscription method after so users will disappear and I can eventually change settings or quizzes. I suppose it will also be the case when restoring a 3.8 course into a 3.9 platform?
Gradebook
Import, restore or copy gradebook setup
This discussion has been locked so you can no longer reply to it.