I actually think you might have an easier time selling something that used to be free. In any case I certainly don't think users are less likely to buy something that was free. If it's free first, users have already seen it so they know what it's worth.
You're arguing that there's a set of people who would have been willing to pay for something had they not been given it for free first. But because it was free, all of a sudden they don't want it anymore once it costs money. I think that's a stretch.
Also consider joe video-uploader who had never thought of paying for something like analytics, but since he's been using it for free and knows how valuable it is to him he can make a better informed decision as to whether it's worth $10/mo or whatever.