You can do that. As long as Jams are still running, this is no problem. Only after the submission period is over, most competetive Jams do not allow you to post any updates within the voting period. Not exactly sure, how IMGJ is handling that, though.
Viewing post in Can you post updates even while you already submitted the game?
You are correct, There is no updating of your game during the rating period. This is for one week. In the case of Improve My Game Jam 29 from the 29th of September to the 6th of October.