so it's my understanding that it's not really known why chang'e initially drank the hourai elixir, but i was thinking about it and came up with what i think is a pretty good headcanon for it. (assuming i am not contradicting established order of events).
she drank it because she got spooked once she realized how absolutely fucking off her rocker with rage junko got about her husband houyi killing junko's son. junko can just kill her anyway because of how obscenely overpowered her ability is (permanently, unless i'm mistaken), but now she has the benefit of being protected while being imprisoned by the lunarians.
junko really is just that fucking terrifying.