I believe that "upgrade" versions can be installed over the existing programme, providing it is of recent vintage e.g. NIS 2005 Upgrade could be installed over 2004. However, as I understand it older versions have to be removed first. If you have a full 2005 version it would be safer to entirely uninstal 2002 whilst you are offline, instal 2005 offline then go online to register it and get updates. You will probably see a big difference in the interface between the 2 versions anyway.
Norton isn't always well regarded in this and other forums but I use NIS 2004/2005 on 4 machines and have no problem. Personally I don't mind paying for an integrated solution which provides updates automatically. When you get to renewal tho', check out the latest version as - strangely - new versions can be obtained more cheaply than renewing an existing version.
Good luck !