I am thinking about starting a vegan diet.
Have fun with that. Veganism is totally abstaining from any animal products. 
I've done the vegetarian thing before, but never full vegan. I'm doing it for health reasons. Are there other reasons to become vegan?
None that I can think of. You basically can't even eat eggs or drink milk. 
Yeah, it'll be a big change. But the benefits will outweigh the negatives, hopefully.
Supposedly it helps reduce the risk of heart disease if you plan well. 
Are there people that shouldn't eat a vegan diet?
, New Zealand Ministry of Health, Harvard Medical School, and the British Dietetic Association. The German Society for Nutrition does not recommend vegan diets for children or adolescents, or during pregnancy and breastfeeding. In preliminary clinical research, vegan diets lowered the risk of type 2 diabetes, high blood pressure, obesity, and ischemic heart disease. Vegan diets tend to be higher in dietary fiber, magnesium, folic acid, vitamin C, vitamin E, iron, and phytochemicals; and lower in dietary energy, saturated fat, cholesterol
List of vegans
Veganism involves observing a vegan diet, which is a diet that includes no animals or animal products of any kind. It can extend to an entire lifestyle that precludes animal products from all aspects of life. Besides food, vegans may also choose to avoid clothes that use products such as leather and fur, cosmetics that have been tested on animals, and blood sports such as bullfighting and fox hunting. People who adopt veganism for ethical reasons may avoid or boycott all products or activities whose production or undertaking is