You can't update an app signed with key X to an app (.apk) signed with key Y. The package names must be different, otherwise you get nasty error, and you canno

You can't update an app signed with key X to an app (.apk) signed with key Y. The package names must be different, otherwise you get nasty error, and you cannot update. Their users would be forced to uninstall the old app and reinstall the new one.