People already have plenty of evidence; they just need a social reason to change their filter.

What I'd say is different between conversions and de-conversions is that the social incentive for religious conversion often takes the form of peer pressure or wanting to belong to a particular group, while de-conversion seems to just require evidence that you won't be completely cut off from good/cool/interesting people if you turn atheist.

I should say that the main evidence I have here is my own history (one data point in each set) as well as a few observed de-conversions in my friends (and one aborted de-conversion from a person who couldn't imagine letting go of his devout family and friends). And I've known people who converted (before I met them) to religion for reasons that sounded more like an innate yearning for deontology.