All Network Inventory Advisor updates are free for the same version purchased
For example, if you bought version 3.8 then all versions 3.xx, which are upgrades to your purchased version will be free of charge, the registration keys provided will work throughout that version, for example version 3.9, all updates can be download electronically from the website any time.
Version 4.0, released after versions 3.xx (for example) is considered to be a new product, this will incur an upgrade for a partial charge.
There’s a special grace period for registered users of Network Inventory Advisor 3.8+ which started on July 1st, 2011. Everyone who purchased Network Inventory Advisor after that date will get a free upgrade to Network Inventory Advisor 4.