Changing App Name doesn't create any problem while you signing the application. you might have entered a wrong password or keystore-alias.
And you cannot create a new keystore to sign the application. If you done so then it is another application and you cannot updated the currently existing application.
Try re-sign the app with First Keystore with proper keystore-alias and password. It may be hard for you but if you fail to re-sign with the first keystore file then you cannot update the app with second version.
