I have a windows store app which I built about two years ago. I released it as a free app, but now I'm thinking about switching to a trial / paid model. After the trial expires, the app would return to the free functionality and the user could pay to upgrade
My question is, can I replace a free app with a paid one?
By replace I mean, change the existing app to require payment, and keep the reviews etc. Is this possible?
Yes. You will need to follow the normal update process. Your new update should have the paid/trial functionality and when you upload the new version you can set a price. You can find help here http://msdn.microsoft.com/en-gb/library/windows/apps/hh694065.aspx. If you want existing users to buy the app then you can look at in app purchasing.