There aren't any legal issues related to switching from free to paid. Just because it was free doesn't mean you must always supply it for free (unless you entered into a legal agreement to do so).

The main issue would be business related. How do you get people who have played for free to start paying. The most common way is to introduce a load of new features when you switch from beta 0.98 to v1.0 and to offer discounts to existing beta testers. Alternatively introduce value added features (as GDNet do with GDNet+) so people can play for free but buy added extras.