I don't think you can damage your body by not preparing for a fast.
I believe you can make the first few days of the fast more pleasant by preparing, but I personally haven't noticed much of a difference. I've done it both ways.
I'll prove my point by a simple illustration. When an animal gets sick or injured, what does it do? Does it eat raw for a few days to get ready for the fast? No. It just starts fasting.