To expand on the how-to-revert, though:
Reverting a Savegame
If you want to try forcing a savegame reversion between any two versions, you should open up the savegame in a plain text editor like Notepad or Notepad++ (not Wordpad). The very first line contains the version number under which the savegame was created. You can manually change that to any older version number. As long as the version number you set is lower than or equal to the version of the game in which you want to load the save, it will at least attempt to load it.
If the savegame format has not been altered between the two versions, it will revert completely successfully. If something has changed, as happened between 2.0 and 3.0 for instance, it will throw all manner of errors and probably dump you into a bunch of empty planets. If that's what happens, the savegame is simply not compatible with the older version of the game you are trying to load it in.
Reverting Settings.dat
There is nothing you can specifically do to revert settings.dat, but the game will always try to load it. It is fairly rare for versions of the game to break backwards-compatibility with this, although that also did happen between 2.0 and 3.0. If you try to load an older version of the game with an incompatible settings.dat, the game presently will crash on startup. Simply removing or renaming the settings.dat will make the game recreate a new one. You will have to reenter any license key(s), profile information, settings preferences, and so on. Your achievements and high scores will also be gone and will have to be re-won. But, as I said, it's farily unusual for releases to make breaking changes with Settings.dat. The only time they cause a break is when something was present and needed in the settings.dat of an older version, but it was removed in a newer version. Most of the times changes to settings.dat are additions, so this is what makes settings.dat more commonly revertable.
Upgrading Is Always Okay
No matter what version of the game you had a savegame created in, or a settings.dat created in, upgrading is always forward-compatible. I still have some savegames from April 2009, before the game was even released, that I load up very commonly for testing in general, but also to verify that the forward-compatibility is working properly.
Anyway, that's the scoop -- hope that helps. Apologies for the briefer version before, but I was out and typing on my phone. I tend to be much briefer then, the iPhone keys are great but it is much slower to type.
