Hi, Dave. Yes, you can simply add OneCare to the new PC per the above. Optionally, make one of the PCs into a Hub in a OneCare Circle and the PCs on your subscription will be listed in the Circle on that PC. You can then expand the details for the PC that is no longer running and select the link to remove the PC from the Circle. This is optional, though. As long as it isn't running on more than 3 PCs you are within the terms of the license agreement.
-steve