I've been wondering that too, especially for the building events since I want to get a head start on those. Correct me if I'm wrong but normally, the big events like Anatomy and Physiology, Astronomy, Chem Lab, Rocks and Minerals, Write It Do It, etc. don't get cycled out. On the other hand smaller events like Water Quality, Geo-logic Mapping, Boomilever, might get exchanged for something else (or in the case of Boomilever something REAL similar).
A good indicator is the schedule for the Science Olympiad summer institute thing. They normally have presentations for the events they're considering for the year and by the end of the week they should have a good idea of what's on the list. I'm sure someone around here has the link to that.
2012 Season: Reach for the Stars-3rd (State) / Keep the Heat-19th (State)
2013 Season: Astronomy-2nd (State) / Disease Detectives-15th (State)
2014 Season: Astronomy-1st (State) / Experimental Design-20th (State)/ Anatomy and Physiology-16th (State)
2015 Season: Astronomy