Originally Posted by ConciergeMike

Yes and no. You can call within 24 hours of your scheduled departure, but the flight you are trying to switch to has to be within 12 hours of the scheduled departure. In short, start calling at T-24, not T-12. Oh yeah, forgot: in addition to the timing rules, the seat on the flight you are switching to must be in the same fare bucket. Likely not a problem for BF, but I just called about this yesterday, so the rules are fresh in my mind.