I know how much you all love to read about poop, and since the wombat hasn't composed any great shit stories yet (she has plenty let me assure you) I can speak about it from experience now.
The first time I embarked on this journey I did the full prep week before I started juicing and had very few adverse effects (of the digestive type) when the fast started. Today, after overindulging in dairy several days in a row the first several juices are doing a thorough and rapid cleaning. I'm not getting any huge bms, which is kind of a bummer... I'd love for it to be all over that quickly, but instead I've had to poop at least 6 times today and it's not pretty. Lots of crampy bowel business too. Blargh.
Overall lesson? Ease in and it will be easier. Jump right in and you will feel it!