My parents have an old non-HD sky box and don't get any HD channels.
If I give them one of my old HD Sky boxes (I have three!) and they put their viewing card in it, will they be able to use it as normal (recording, pausing, etc) and get BBC and ITV in HD?
When I moved house and found an old HD box here, I just phoned sky and they paired my old sky + card that was used in my old sky + multi room box to the newly found had box.
It should work ok, you will even get some hd channels for free (bbc ,ch4 and some of the itv channels). there may be some functions that don't work but best thing is to pair it to the box. I did mine via online chat with sky, saves being on the phone for ages!! took about 10 mins.