Well,
It's not like the workout is changed, but actually limited. Many of Cathe's DVDs last over an hour, and FitTV only airs 1 hour programs, including commercials. This means you end up doing about 45 minutes of the actual workout... Which results in several exercises or step combinations being cut out.
I mean, if you're just beginning working out with Cathe, they are much better workout than any other show there (in my opinion). But once you get to a better fitness level, you're able to take the whole enchilada by doing the shows in DVD. Just an example: Boot camp is only 6 circuits long on FitTV, while the real deal lasts for ... 8 circuits!!! It may not sound like much, but try doing those 1-minute excercises, and you'll understand
So, FitTV shows are a good intro... but once you start buying DVD's,,, you won't be able to stop! They are so great, plus the premixes make them feel like you've got tons of excercise routines in one single DVD.
I'm sure others will have more info on this... But my overall opinon is that, even though you can get the FitTV versions Tivoed or taped, getting the DVDs is sooo worth it.
Have fun!