Maybe horsy could have it that if you had originally agreed to do the 200 challenge (the ones who had posted before the original date in the challenge thread), you still qualify for the reward you had been promised, since there was no deadline to have to complete it by. It would only apply to those people and anyone who had not originally agreed to it in the first round would have to abide by the new rules?
I mean I know some would see that it would be unfair that they have to do 800 vs. someone doing 200, but you are agreeing to different terms (completing 800) than those who had agreed to the original terms (doing 200). That way those who had begun the 200 challenge but not completed it could still get what they had been promised as a reward from what they had understood as being a project with no time limit, and the new challenge goers will know how many they need to do under the new agreement guidelines?
Just a suggestion, please don't eat me..