My travel agent keeps mentioning there may be upgrades offered to past guests. Just wondering how often that actually happens. Anyone with experience there?

At this time Carnival is running a past guest upgrade for new bookings. Have you already booked your cruise as of yet? This upgrade is limited to certain catagories and new bookings only. An example of the upgrade is: you book a 6A you will be upgraded to a 6C, and so forth.

Everytime I've tried an upgrade (well only twice) I've been told it's NOT available. I don't quite get it.

you need to call her every week before the cruise at 5 weeks out to have her check for you, we have gotten some great upgrades for lower prices amonth out from our cruise date ,it depends on how she fill ups with pax.