IANAPE (I am not a petrochemical engineer ) but I believe that the statement is correct but doesn't tell the whole story. As far as the actual distillation process is concerned, there is a fixed amount of diesel in each barrel of crude. However, after that step, there are additional chemical processing stages that can convert one product into another. I know that refiners do this for gasoline to increase the yield of gasoline from each barrel beyond the fixed percentage you'd get just from straight distillation, so I don't see why it wouldn't work with diesel as well. However, you'd probably have to spend a lot of time and money building new processing facilities to handle a 50% spike in diesel demand if so many people switched over.