From what I read it sounds like you can tweak the game page a bunch to add screenshots and videos and whatnot. But the upload time is sacred and I imagine if you reuploaded after it was over you might get disqualified. This is my first jam in decades though so I don't know all the rules. :)
The game state is actually locked so there's no danger of unintentionally breaking the rules . At best what you can do is remove yourself from the jam and then you can do whatever you want, but obviously this blocks you from adding yourself back in. Totally fair. I haven't participated in many of these so I am still learning the rules and ropes .