That certainly strikes me as a valid interpretation, and indeed, I think is the "baseline" that gives a lot of the rest of it its charge. On the other hand, another take might be that the first shape change is into a snake, associated with cthonic death and rebirth. Going underground might be getting buried, or going to the underworld. Having sex with an underground female figure might be a symbolic representation of death, the precious liquid is something only available "out of this life," and transforming into the eagle is escaping from the usual fate of the dead to something else.