Android :: Generating Device Specific Serial Number

I would like to be able to generate unique serial numbers for each Android device for use in unlocking an application. How could I do this? EDIT: The reason is I want to revamp a paid application and provide users who have paid for the old version, which will have a different package name, a way to obtain the full version by downloading an unlikable free version of the application. I would push an update to the old version that would generate and display the code that they could enter to turn the Free version into a fully functional version.