Sometimes, academics have to change completion criteria for a course after students have already completed some criteria for the course. There is a warning message saying that Unlocking the completion criteria settings will delete any existing user data and may cause confusion, however, sometimes there is a demand to do so.

After "Unlock completion options and delete user completion data" button is clicked Moodle deletes course completion data, including activity completion dates (but not completion statuses). So before the "\core\task\completion_regular_task" scheduled task fills out criteria completion dates we can see that completion report (/report/completion/index.php?course=N) shows the following:

1. Empty/not completed course status and date for each user (this is expected),
2. Correct activity completion states, so if the activity is completed it will still be displayed as completed (this is expected),
3. Empty date of completion for completed activities, so activity is marked as completed, but has no date (this date what would be good to preserve).

Once the "\core\task\completion_regular_task" scheduled task is completed, the report shows the following:

1. Course completion state if completed has a current date, the date when the task did its job. This is expected as we changed completion criteria and don't care about preserving old ccourse completion date because old completion criteria is not relevant anymore,
2. Completed activities have a current date too, while they were completed some time ago. Setting this to the current date is not really useful, because it doesn't reflect what really happened - activity wasn't completed today. Yes, criterion was completed today, but it would make more sense for end users to show activity completion date, when an activity was completed.