Once you download the paid version you will need to switch to that version of the app. The paid version has a white border on the app icon.
You can first tap the small text at the bottom of the login page and try entering your word chums username there. If that does not work you can just login with the Facebook or email account you put on your word chums account. If its email be sure to tap Existing User after tapping email.
After you are logged in to the paid version you can delete the free version from your phone (black border around icon) Thanks for supporting the game and spreading the word!
The paid version has blue paper on the icon and the free version has green.