No, we've always followed a policy of allowing you to re-use your old key. You are restricted to the features that were available with the key that was issued to you, though. Upgrades are only necessary if you want any of the new features.
When upgrading to 6 there are a few things that you need to be aware of.
First, it requires license activation with our servers.
Secondly, when upgrading it will prompt you to backup your current settings. This is recommend.
Third, after the upgrade you should pay particular attention to the following areas:
1. Ensure that your adapters are detected correctly - this means your internal adapter should be detected as internal, etc.
2. Ensure that your bindings are still correct. WinGate 6 uses binding policies, which means it can adapt to en/disabling / removal of adapters much more intelligently.
Fourth, the UI has changed a bit. You'll find more information in the various panes as well as a new look to the dialogs which makes it much easier to navigate around tab-heavy dialogs.
Fifth, if you are using the mixed user database option (WinGate or OS) this has been removed. You now have to choose which one you want to use. (This affects which authentication options are available).
That is just a rough overview, though. You can find a full list of features and changes
here.