A very interesting question Harold. As you know pumpernichel flour is just coarsely ground dark rye flour. When changing rye flours
you will get a slightly variance in color as well as taste in the result. I am sure you understand that the hydration amount varies as well…the darker the color you will need a bit more hydration.
I do this between light rye & medium rye. There is no more problem than what I just mentioned Harold. Good luck in your bread baking.