If you already have a satellite dish, switching to Freesat is easy. Freesat uses the same dish and satellites as Sky, so if you've already got a dish all you need to do is connect a Freesat box or TV and away you go.
You'll receive over 200 TV & radio channels, including 11 in HD, all subscription free and if you choose a Freesat+ HD box, you'll be able to Pause, Record & Rewind live TV and watch TV on Demand with no monthly bills. Remember that your dish will need two feeds/cables to your box in order to watch one channel while recording another - if it doesn't, you can arrange for an installer to visit when you buy your Freesat box.
If you have a warranty agreement for your Sky dish (typically 12 months), then check the terms before you install any other equipment. Also remember that if you have any remaining contractual obligations with Sky, you'll still be bound to fulfil them.