I'm not sure if I'm reading this correctly but this might also be one of those cases where the English language didn't help in the slightest:

"Red liquorice" is not what a German/Dutch/Scandinavian person would call liquorice. It's just red soft candy. It doesn't contain the same active ingredients.

So "switching from red to black liquorice" is in fact just a complete switch of diet, not a slight change at all.