I am living proof that nothing "terrible" happens to a person who quits eating meat - as I did almost 40 years ago after having been raised on meat! I didn't resume eating meat once my cancer was gone, but once in a while (like, once every few months) I enjoy a small amount of poultry.
I've continued to eat fish (not shrimp) once or twice a week. I still avoid eggs and dairy products. A few years ago a friend who is a chiropractor urged me to try the "Eat Right for Your Type" diet and since my blood is Type O, I'm supposed to be one of those who needs and can handle animal flesh. Well, after a week of eating good quality meat once a day in small quantity, my legs swelled up to double their normal size. I dropped the stupid diet and the swelling went away, never to return.
I think the farther away we eat on the evolutionary scale (food chain), the better off we are. In other words, plants are better for us than animals, in general, and if we eat flesh, fish are less harmful than red-blooded creatures. And among red-blooded creatures, birds are farther from us than anything that walks on four legs.