Well, I logged on to the page you talk about and don't see any "modify booking" button. Since I didn't book directly with Carnival the info may be different?? I wouldn't think so but who knows??
In the past I have called Carnival a few days before departure. Sometime they could provide the cabin number and other times they told me it wouldn't be available until I got to the pier. Guess I'll just have to wait.
Thanks for your help, though.