There was no need to buy the application again since it's a mobile one. You should have logged on with the email used to purchase the application and then Wordfeud would be available for downloading again without issues.
Username is irrelevant for mobile applications, because the email is the one that counts. In the future, login with the email and both the progress and the name should be restored automatically, only if the software supports backing up to Facebook, Google+ or their own service.