If you mean that you wish to unsubscribe OneCare on a PC from subscription "A" with one LiveID and re-subscribe it with a different LiveID, simply make another PC that is subscribed with LiveID "A" into a Hub PC and then expand the details for the computer you wish to change in Manage Your OneCare Circle on the Hub PC and select the link to remove the PC from the subscription.
If you mean that you want to transfer your OneCare subscription to a new LiveID, that can't be done, to the best of my knowledge. When your subscription is due to be renewed, let it expire or contact support to cancel. Subscribe with the new LiveID after reinstalling OneCare.
-steve