NAV 2002 is fine, so keep it till advised by Symantec to upgrade.
Time to upgrade is when Symantec stops the technical support for your version.
For NAV 5.0 and NAV2000 6.0 & NAV2001 7.0 technical support has stopped. So the advise is to upgrade.
NAV5.o can not be renewed.
NAV 2000 6.0 can be renewed. Advise upgrade
NAV 2001 7.0 can be renewed. Advise upgrade
If you still have win95, you cannot upgrade,upgrades won't work on win 95.