On a spiritual level, a new world, highly polarized in light instead of darkness (i.e. the opposite of what we have today) will mean that these 'people' won't be able to live in it, let alone thrive.

Kind of like garlic for a vampire.

These demons thrive in our current world because of its dark cycle. In the new world, they will just not be able to. So basically we won't have to do much besides bring the light :).