That does happen but seldom. Carnival has plans for the "new" ship and Holiday. THey do plan years ahead but of course all us "curious" people are dieing to know. I do expect Holiday to be in Mobile until Dec. 2009. That might change but very doubtful.
I also was booked on a sailing back in 2006 and was put on another ship, there were major changes because of Katrina. Ships were used to house workers and then the Sensation had issues with her stabilizers. We actually sailed Fantasy from Port canaveral on a 5 night visiting Half Moon and Nassau(edited to add- I forgot Grand Turk). 
I do agree Debbie, if you are booked Holiday in 2009 and get Fantasy (with the evolutions of fun upgrades) that would be awesome. 
Doby Travel
www.DobyTravelOnline.com
Upcoming sailings:
December 13, 2008 Fascination (Mom, my princess, and me)
September 5, 2009 Fascination (open group cruise)
September 19, 2009 Fantasy (open group cruise)
OASIS of the SEAS! Family cruise 2010!