Yes, your body goes back into sugar burning mode (glucose-adapted) very quickly. Pretty much the moment you start consuming anything more than a tiny bit of carbs, your body will revert to using glucose. This is why it's so important to maintain the ketogenic diet. Think of glucose as the body's "go to" energy source, the moment it senses any is available it leaves fat-burning mode so that it can use the glucose and save the fat for use at a later time. You can't really store glucose, so your body prefers to use that and store fat basically in case of emergencies.

There is little evidence of long-terms effects, though there are early indications that it will help fight cancer and age-related degenerative brain diseases. As far as downsides, there are risks associated with doing this improperly. If you are healthy and consume a healthy diet (providing all the vital nutrients humans require) that is ketogenic, the potential for long term harm seems negligible. If you are ill then the diet can be dangerous, so obviously one should always consult with a medical or health professional before embarking on something like this, whether you're healthy or not. I am not licensed professional and I am not giving medical advice.