As I understand it, adding new features after the submission deadline is generally frowned upon, but bug fixes are sometimes allowed. It's like cheating.
A pretty normal rule is thay you would need to keep the original version available for any updates for comparrison so that it can be seen if new features were added.
Most jams are not managed by big teams that can check to see if you followed the rules, so I guess it would be up to you. However the people that vote on your game in the jam, might take any updates into account, or only play the initial jam submission.
After the voting period you can do whatever you want without concern, it is your game after all.