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.
This is definitely true. I made my game free and it jumped to the top 100 lists in a few countries overnight due to bunch of those automated sites that list app "sales" picking it up.
