I don't think it would be a huge mess at all - people could simply just withdraw their entry from the group and submit a new one. Or "update" their entry before the contest closes for judging (though I suppose they could do that anyway lol). I know that deadlines get extended quite frequently, but they also apply to everyone. I'm sorry if you disagree - I'm not trying to start a fuss - but that's my opinion.
If we did this, people who would have to withdraw their submission, would only have 15 days to submit their new piece, whereas they had 30 days initially to submit theirs previously. Thus, I do not see how this would help anyone.
15 days is 15 days. Any extra time is always helpful =]