An interesting question is what happens to the buying and selling of Bitcoins. How long will transaction confirmations take then? If traders are forced to pay large transaction fees just to get a reasonable confirmation period, Bitcoin will be less attractive to use.
It would take one diff adjustment where transaction processing took longer, then after the adjustment it would be back to normal. The whole idea of the diff is to maintain a 10-minute block rate, right? If that's true, then if the diff drops big it should re-adjust downward enough to be able to return to a 10-minute block finding rate, thus transactions go back to normal.