Google Play: You can't charge for previously published free apps
If you've decided that you'd like to put your new application in Google Play, you'll need to decide whether you're planning to make the application free, or charge for it. This initial decision is fairly important given the rules of Google Play.
If you publish your application and make it free, you cannot at a later date start charging for the same application. This is one of the rules of Google Play. If you would like to start charging for the application, you need to create a new project in Andromo that contains the same information, and upload that to the market as a new application. (You may use the same name and information in Andromo.) You can also use this method to provide free and paid versions of an app that may be slightly different.
This rule also applies to applications you previously charged money for. Once they've been made free, you cannot start charging for them again. You will again be forced to create a new project.
Please note that these are the rules of Google Play. We didn't make the rules - we're just telling you what they are.