That's a fascinating question! Do you know many vegans? I know a lot of them, and I'd say on the order of 75% of them went directly from omnivore to vegan.
They hadn't thought about the ethics of meat before, when they did, they chose to be vegan.
So it turns out (in my experience) telling rational people "do the thing that makes sense, even if it's a little extreme" is much more likely to result in the most extreme change of behavior. But we're deep into anecdotal/psychological theory land here, so I claim no authority beyond N=30 vegans.
