If you've got Sky already, you might struggle to get a deal. They might just say no, you have to pay full price.
On www.avforums.com (recommend this site!), they say if you want to upgrade and they won't play ball with a deal, then phone them up and say "I want to cancel" and ask to be put through to the cancellations department. When they ask why you want to cancel, say you've been a customer for x amount of years, and you're mad about all the deals they have for new customers.
With luck they'll give in straight away. If they don't, cancel anyway and hopefully they'll contact you. If they don't and you really don't want to lose sky, then ring back before the cancellation date and reinstate yourself!