Not quite sold as an upgrade. Anybody, even somebody using a version 3 license key purchased 6 years ago can use WinGate 6.0. The featureset is restricted to what you had at the time you purchased your license, but you get all the bug fixes.
So, using your purchased version 5.x license with version 6 will give you the same functionality you have currently with the UI improvements, stability improvements and bug fixes since 5.2.3.
If you want access to the
new features (Bandwidth control, etc.), then you will need to check the upgrade pricing. You can verify the price of an upgrade by entering your key information on the purchase page. You can check which features are available on which key type and version here
http://www.wingate.com/wingate-licensing.php
Again, however, depending on when you purchased your version 5.x license you might be given a free upgrade to a version 6.0 license, which will include the new features.
The unlimited versions have had a small name change, they are now called
WinGate 6.0 Professional 250+ concurrent users
WinGate 6.0 Enterprise 250+ concurrent users