If you simply stop paying your subscription, your Sky box (with the old card in it) will continue to receive all of the channels listed here:
http://en.wikipedia.org/wiki/List_of_free-to-air_channels_at_28%C2%B0E
So you might not need to do anything at all.
Freesat offers more channels than Freeview:
http://www.freesat.co.uk/what-you-get/our-channels
but you might not actually want any of those extra channels anyway. ('GOD', 'Islam Channel', Psychic Today', anyone?).
Some of the channels owned by UKTV (such as Dave and Yesterday), which I particularly enjoy, aren't available on subscription-free satellite TV. For them, you need Freeview:
http://www.freeview.co.uk/Channels
Chris