It is definitely something your body will need time to adapt to and should be eased into. (I think the Getting Started guide has some instructions for this). How much you will need to ease into it will depend on the person and a lot on what their previous diet was.
Regarding the flavors, be aware that they contain inulin which is another substance that can cause GI issues including diarrhea. It occurs naturally in foods and has some beneficial properties, but is again something your body may not be accustomed.