We just upgraded from a regular interior to a Havana interior on our May 14th Celebration cruise. The upgrade we were offered was from Interior to Interior(higher deck), Cloud 9 Spa interior, or Havana interior. We took the Havana interior because we liked the extra access to the bar and pool in the Havana area. It came to my email and was in the "Manage My Booking" section on Carnival's website.
After reading this thread I've got my fingers crossed for a last minute phone call in the spring offering a balcony upgrade, lol.
EDIT: We booked in November, got the upgrade offer a week or two ago, a little over 100 days from embarkation.