I rang Sky up for the same reason. I was told that at the end of July, ESPN will stop and if I wanted BT vision, I had to go and subscribe on the BT website. It won't happen automatically. My monthly subscribtion fee will be reduced by £10.
I am not subscribing because there is also a one off fee to pay for BT vision which I think is £50, non refundable.
If you have BT broadband, the service is free and you still need to subscribe.