If you change it from 100 to 250 then think about it.
If you already moved everyone to the group with over 100 posts and you change it to 250. Then all the existing people under 250 but with at least 100 posts will remain in the group. The software will not delete people under 250 they are already in the group.
So you would use the same method that you added members, to delete them. You would just change the search to less than 250. But I would think that would confuse members that once upon a time they could post now they can't.
Of course the members that are currently under the limits that never were in the group will migrate on a natural order.
