I read somewhere that if you pillage a province down you can switch it to humans. I tried this and after pillaging an orc province all the way down I received 3 options: salt, curse, do nothing. Tried all 3 and none of them changed the population type to human, it just made dead land, cursed land, or no change while keeping the population type orc.

If it is actually possible to change a province from orc to human can anyone explain how?

Impossible.