I was going to say "switch
into the I fund" (or whichever between C, S and I seems to be getting hammered most), or at least, "leave it alone," but then you wrote this:
I hear that if you make any changes before noon EST, you won't be affected by today's massacre.
Are you saying that if you switch to the G fund within the next half hour you'll get
yesterday's prices? That's crazy; it's like being able to see the future! If that's truly the case, then you could do something like switch to the G fund now to lock in the gains, then switch again to the I fund after noon (or next week) to buy the dip. Assuming it's allowed and you have gigantic risk tolerance for market-timing/speculation/arbitrage, of course.