Interesting. I've never really understood how someone could do it just for health because I think it would be so easy to make exceptions. Your health isn't really going to deteriorate if you have a slice of pepperoni pizza once a month for example. I'm not trying to criticize health vegans; it's just a rationale that's hard for me to fathom.
It's an identity level change. Another example for that would be smoking. Couple years ago I used to smoke, today I wouldn't touch that crap. When you have a certain self image in your head it will make your behavior match that self image.
I stopped eating junk food and stuff like pizza 2 years before even discovering Vegan Nutrition. The switch for me was much easier than many ethical Vegans I saw. Mainly cause I believe I just did some more replacements to a more healthier Diet.
I don't believe in moderation when it comes to stuff that I think they are junk for my body.
Western science can't see the whole process that creates bad health before something happens. But the process is there. So I don't need to wait for something to happen. I believe in prevention and I refuse to put in my system any foods that I think to be far from natural and human body friendly. So it's very simple when you make a perception switch of that kind. There are some stuff that I used to eat and I don't even see them as food anymore. I used to drink bottle of coke everyday. Today coke is no more than a bottle of poison for me. We learn to eat certain foods and avoid others so we can unlearn it.