Speaking unofficially, the jam submission form will allow you to do either.
I'll be updating my existing page, but I could see some folks wanting to keep a version of their original game as is, and so making a new separate page.
In my case, if I have time to participate (this weekend is… complicated), I will update the project, but I will keep the previous version as downloadable, so that anyone who wants to see the differences can do so.
