Moving this to the General topic folder of the forum....
You cannot change your subscription LiveID for OneCare. The ID is your Billing account and the only way that I am aware of that you can change this is to cancel the subscription (losing the balance of the period) or let it lapse and then create a new one. So, to add a PC to your Circle or perform any management tasks in OneCare that require a LiveID, you must use the original Subscription LiveID.
You can contact support to confirm this:
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve