The New York Times offers a summary on the difficulties of converting a free service to paid. Coca Cola did it with water, but newspapers are having a harder time convincing consumers to pay for their online content. The debate continues over paid-vs-free online content, with the Wall Street Journal and the Financial Times charging for some of their features, while the New York Times and the Los Angeles TImes both reverted to free content after their experimentations with a paywall did not bear fruit.
You can read about paywall history in this New York Times article.