When I talked about not being able to change the nature of Samsara, I meant that sometimes in one's efforts to heal Dukkha in the world there comes a certain amount of idealism that we have the power, personally, to make all Dukkha disappear from the world. However, the nature of the cycle of Samsara, birth and death, is not under our power to change. And one of the characteristics of existence is Dukkha. We can lessen the Dukkha in the world, but being able to change the laws of existence is beyond our power. So we can lessen Dukkha, but still some things in life are going to be unsatisfactory.
Part of the unsatisfactory nature of Samsara is that we have a body that consumes things in order to survive. In maintaining bodies there is a lot of death and destruction in the world. We only have to live in the forest here for a little while to see this unsatisfactory nature of Samsara, and the suffering that comes from needing to support a body. So materially it is very difficult to end all Dukkha in life. Early on before we met Buddhism, there was a bit of a struggle with us trying to figure out how we were going to stop killing things, and we went to rather extremes in trying to find out how to do this, but we eventually gave up thinking in this way as there is no way to do it materially. The body needs food, clothes, shelter, etc. and because of this, other beings die, rather we have to transcend Dukkha. So we were very ripe for Buddhism when it came along because we had realized that we can't actually change the nature of Samsara. Because of bodies and the need to support them, there will be some Dukkha. So that's what I meant.