I've been a sky customer in the past and since i cancelled they keep calling to try and get me back. I only ever had normal sky digi box, but it seems they are willing to offer me a free upgrade to sky+ or sky+ HD to return. If i can get a free upgrde to the HD box out of them does it cost more to receive the HD channels such as sky one hd, then on a normal sky+ subscription.
As far as I'm aware your basic subscription to channels should remain the same price, but you will NEED the SkyHD Pack, which gives about 26 channels in HD for an extra �10 per month (check the Sky website for details on these), in order to get HD channels.
Its a good start if they are willing to give you a box and do an install for free!
But remember you will need an HD ready TV set and and HDMI cable to connect the new SkyHD box to the TV to actually get the benefit of HD.