Your existing viewing card will be used in the HD box.
You can always talk the installer into running cables into another room (bacon sandwich, price of a few pints etc) and plug your Sky+ box in.
You won't be able to watch much though without a viewing card.
You can get a 'Freesat from Sky' card from sky (oddly enough
) for about £20. This will give you the channels shown here
freesat from sky list
However you won't be able to use the sky+ part of your box (ie no recording or live pause etc). For that you would need multiroom - £10 a month. This would give you all your channels that you have in your subscription (except the HD ones).
Alternatively you could go and by a Freesat box for 50 quid or 280 quid (if you want it to work like sky plus) - this doesn't need a subscription, and gives you the channels
Listed Here
Hope this helps,
James