sky emailed me back and said this:
Did you know that as an existing HD customer, you are already eligible for all of the exciting features introduced with the new Sky Entertainment Extra+ pack.
Your current HD subscription gives you the very best of Sky TV, including access to:
Up to 67 HD channels
Must see TV Boxsets on demand
Sky Sports F1®HD
Switching to Sky Entertainment Extra+ may mean that you could miss out on some these key features. To provide a better service we would recommend, in this instance, that you discuss your package options with a member of our customer service team on 0844 241 1818.