I just discovered this to my shock and horror (Moodle 2.8 here).
I do not care what the intention of the design was, but the default HAS to be to lock the student submissions after the due date. I have now acquired years of experience with Moodle. Even I did not know this highly unacceptable, and downright dangerous default. Let alone newbie teachers.
Seems you have to set an additional cut off date to prevent this from happening. This is downright stupid. If you want to have the frills of a cut off date, the default should be to lock that date to the due date, and ask teachers to disable / change that if they need to. Most teachers would not know or understand the difference between cut off and due date.
In real life, after a student submits an assignment to a teacher in paper form, they do not get to waltz into his or her office and merrily replace it with anything else without the teacher's permission (which would rarely, if never, be granted).
Please fix this ASAP.
No student should be able to edit / change anything after the due date passes.
I mean, was someone high when they came up with this default???