Getting back to the OP, each SC that ships theoretically takes the place of what, 50 singles?? I really doubt that ratio works out to a drop in difficulty. Not to mention that jalapenos will be shipping too, each taking the place of 5 singles. In BFL timeframe, we'll see an initial launch in December, right around the reward halving. Logic would suggest that people won't turn off their GPU rigs until after both of those things happen.
I think you are missing the OP's point here. Of course there will ultimately be a difficulty jump for the reasons you give. But if BFL requires everyone to ship their old singles and MRs back *before* their SC hardware is released, this would lower the network hashrate (and difficulty) short term. So the discussion of whether or not BFL will require that they have the old hardware "in hand" before they ship the new hardware is entirely relevant to the OP.
