"but I can't find a way of moving to 2013 free version."
For XP, go to Control Panel -> Add/Remove Programs. For Vista, 7 or 8, go to Control Panel -> Programs & Features. Locate "AVG Internet Security" in the list, click on it then click the "Change" button. You'll be presented with a list of options and one of them is to downgrade to AVG Free edition. The process is automatic and usually takes such a few minutes.