What this poster on "Maidan" meant is that the angelic powers are "pure spirit", and not "pure spirits". "Spirit" is the only component making up their being. This doesn't mean, however, that they are "unchangeable", in the sense that it is impossible for them to reject God. What it means is that, unlike us, when angelic powers make a spiritual choice, they do it "whole hog". People are not "pure spirit". Unlike angels, we have bodies, and many would say other components in our nature that angels don't have, as well as a spiritual component. Our bodies constitute a big part of who we are, as well as other parts of our nature. So we can be ambiguous about things. We can accept God, and then go out and sin. I'm not articulating this very well, but: we can be mostly accepting of God and still be somewhat rejecting of him too.....except for the saints, most people who are striving to be in union with God are like this, in varying degrees! Unlike the angels, we are made in the image and likeness of God, so we can never completely reject God. But angels can, in so far as it is possible for a rational creature to separate itself from its creator. So because of their "pure spirit", angels are either completely holy or completely debased (in which case they have become demons). There is no middle ground for them. I don't think it's written in stone either way whether or not demons will repent in the end. That's really none of our business, in a sense.
Angelic powers are certainly "changable". Nobody forced those who rejected God to do so. They chose that state, just as the holy angels chose to remain obedient to God.
There. Clear as mud?