One example of such a site is Meetic, which started as a free dating site for a couple of months and then started charging a subscription.<p>How do you prevent customer backlash? How do you protect yourself against a competitor jumping in with a free alternative?
Tarsnap started out in free beta before moving to paid beta -- but this isn't quite what you mean, since at that point it was still a <i>private</i> beta (tarsnap became publicly available the month after it moved to paid beta).<p>I think as long as you make it clear from the start that your site isn't going to be free forever, you won't get any significant customer backlash -- simply because those people who would provide said backlash wouldn't sign up in the first place. Whether this is an effective business strategy is an open question, but it certainly seems like the most honest approach.
Pandora? They are in a very competitive industry but they were sticky enough that most people didn't go to a competitor.<p>Also Hot or Not. They started as a free dating experiment but when they were overrun by spammers they switched to a paid model. Membership was still free.<p>I don't know if any of that's relevant to you. These are "freemiums." I can't think of any examples off the top of my head on a completely free to paid. Maybe the lesson there is you need to keep something free?
MetaFilter introduced its lifetime $5 membership fee after initially being free. Existing accounts were grandfathered in, which does a lot to prevent backlash. (If I remember right, LiveJournal also grandfathered in existing members when they first introduced paid features.)