it seems many have installed a new version straight over the top, without even bothering to uninstall first
Indeed, if you have unexpired old version, Norton itself via LiveUpdate will often offer to install the latest version over the top for free. That's why on eBay you often see "NAV 2007 upgradable to 2013" for sale. Altho' for a clean install it better to use the Norton Removal Tool (NRT) 1st.
As long as you get the licence key for a relatively recent version you can upgrade to the latest for free.
I have found for NAV it is 2006 onwards and for NIS 2007 onwards. Never having used Norton 360 I can't say what that is but as long as you get your current year or later you'd be OK.
EDIT: Believe this page defines which versions are upgradable to latest: Support Policy
it might be possible with ONLY the key
Just make sure the copy you are buying is brand-new so that the key has never been registered with Norton.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC
Edited by XRaySpeX (Tue 18-Jun-13 14:21:10)