The reason the completion data must be removed is that changing the completion criteria will impact who has completed the course. for example if you had completion criteria saying "must receive a grade of 70%" and you change it to 80%, you could end up in a situation where there are people which a grade of 75% who are marked as complete.

To avoid that the data is removed when changing the criteria - however the completion criteria is recalculated the next time the cron runs and anyone who meets the new criteria will be marked as complete again.

So you your case, anyone who is already fully attended should automatically get marked as complete next time the cron runs, as long as the completion criteria is still met.

Simon