When some students have satisfied Course Completion criteria, the settings are locked with the message "One or more students have already completed a criterion so the settings have been locked. Unlocking the completion criteria settings will delete any existing user data and may cause confusion. "

This gives the impression that unlocking the completion options will permanently delete user status. In fact, according to the FAQ, "When the scheduled task for handling completions next runs, it will recompile the course completion information."

https://docs.moodle.org/36/en/Course_completion_FAQ#I_made_a_mistake_and_had_to_erase_my_student_data._Do_my_students_have_to_redo_the_activities.3F

I think it would be more clear if the warning message is changed to:

"One or more students have already completed a criterion so the settings have been locked. Unlocking the completion criteria settings will delete any existing user data. When the scheduled task for handling completions next runs, it will recompile the course completion information. Course completion information may display incorrectly until this task runs."