I do not think you will be able to revert. I haven't really tried, but I'm not sure that it's possible. You could, of course, simply create a backup and run it in offline mode.
I think the upgrade to BNW is worth it. The trasition is a little rough, from having all those awesome tile improvements to . . . not having them, hehe. Buildings having less emphasis, all numbers be drastically reduced, etc. But once you get over that, you experience the vastly improved overall gameplay of BNW.
Diplomacy is much more tangible and feels like a legitimate playstyle, as opposed to "spam gold endlessly -> bribe all CS -> win." The AI is also much less predictably violent. They take GPT and other things into account now before waging hopeless wars.
NiGHTS doesn't look like it will be updated for a long time, and I personally can't imagine going back to G&K just for the sake of NiGHTS. BNW is just that good. Also, once the Communitas Expansion Pack releases (first full featured release coming up in August, there is a lite beta version currently available but it doesn't change much), I think NiGHTS will be fully obsolete. They have some pretty interesting ideas being tossed around, much as NiGHTS was/is the king of Civ V mods.
