I think you can improve your program when you want.
the important thing to know is that you must make the source code of your contribution available for users.
( You must give it for the contest and it will be diffused to public by dbteam I think )
But you are not obliged to diffuse your medias ( you can keep them secret if you want )
Then, when the competition will be over , you can freely make modification for your program to make it better . . .
And you are not obliged to diffuse modified source code.
I hope I'm not wrong.