The Ferrous and the Rig do have significantly different geometry, so that's something to consider: the Ferrous is non-G2 (built around 38/9mm offset forks, but not quite G1) geometry, while the '08 Rig is G2, with the longer offset (51mm) fork. Swapping parts over from the '08 Rig to the Ferrous will have you putting the Fox fork with the longer offset on a bike with a super-steep headtube (72.6 degrees, if I remember correctly), and this will result in a very quick handling bike. Very.
I have a Ferrous and I really like it, but I can't imagine putting a G2 fork on it. I might consider putting a 44mm offset ('modern' 29er offset) fork on it with the travel bumped up to 100mm to slow it down just a little.
One other thing to note is that the Ferrous is a heavy frame. Between the steel and the chunky EBB it's a bit of a porker. It makes up for that in a very forgiving ride, but you're not getting a weight-weeny bike with it. I've built mine up with reasonable parts (Thomson, Rhythm Comps, etc.), and even as a single speed it's well over 27 pounds.
Good luck on making the switch if you decide to go that route, but remember that more than just the frame material will change.