The Cubase Pro license works with versions as old as Cubase SX 1, but for some reason not any version of Cubase VST 5. If you try to run Cubase VST, it will use your time-limited “All Applications” license instead of your Cubase license.
I think this is a no-brainer. You should be able to run Cubase VST to check how your very old projects were supposed to sound like and which VST plugins you were using, before using Cubase SX 3 to convert these projects into the current .cpr format. I think this would be a good incentive to get very old users to buy Cubase Pro, at least on Windows.