After May 3rd users will get a link where they will post the urls of their project to finalize their submissions. After the finalization period is over, you will not be able to add any new games.
That being said you will be able to update the files at any time just like a normal game. We only ask that your game be in a playable state at the time of submission on May 3rd. For example a blank "coming soon" itch project page wouldn't be okay but anything that can be experienced is okay.
Please let me know if you have any more questions.