The "Upgrade" to a higher Number version eg 3.5- 4.5, 3.5-7.5 doesn't really a upgrade The previous # version. But the new version, installs, as its own separate entity. You don't need previous # versions and sequentially install a higher # version. What you DO need, is the controller. (That's where the software protection is) You MAY get away with installing newer versions, with the 3.5 controller, but that controller is no longer supported.
The controller has changed from the 3.5 days to the InterLink Elite. (with up to date features) If you do upgrade, to say 7.5, and have controller issues, the 3.5 controller, is no longer supported, and not sold separately. Things DO wear out or break. It would suck to have the new software, with no way to use it.
The best advice, I think I can give you, is to buy the whole new setup. That way your covered into the future. And I'v got to say, the support is really top notch, they do take care of you. If you've been using RealFlight, you know its worth it