Addtionally, if/when you make your app free, you'll hopefully get more people to download it. These people may leave you reviews on the app store about what they did like and what the game was lacking... information that you could then use to make it better and maybe learn some ideas for features that you could add to the "premium" version for pay. Additionally, having this 2nd free version of the app could then serve as a marketing tool to promote the "premium" version.