There is some middle point between "at present" and "forever", you know. [She would be the expert. She's only a little curious though, Sebastian is an odd name for a demon in her experience. Weirdly human, but he looks like one of those too.]
I've noticed that- at least for Gods- once they take one form and identity they tend to stay with it for a long time, even by our standards. It's the mortal world that's always changing.