Well, if that was the case, they could probably take a challenge that they intended to be for immunity and just make it for reward. I don't think they'd have to drop or add anything.I suppose they could probably keep the challenge itself the same. They'd still need to come up with a last minute extra reward though.
Interesting side note: In the episode description for the Finale on the TV Guide website it says the episode contains "the final immunity challenge", as if there was only one, but if there was a Final 2 then there would be 2 immunity challenges. It probably doesn't mean much, but if anyone's looking for hope for a Final 3 and a Cirie win, there you have it.
