You can always upgrade to a minor version without a key (e.g. F3.0 to F3.5) but if you jump from F3.x to F4.x (or higher) you would need a key for the version you are upgrading to.
As far as I can see there is no F3.9?
I beleive there was a change in the software around F4 so that if you wanted to upgrade from F3.5 to F9.1 you would have to go via F4.x otherwise the upgrade would fail.
You can always check the release notes here ....
