"Damnit! What went wrong? I thought personal continuity was strictly subjective, and I could redefine it however I wanted!"

Your plan succeeded. Nothing went wrong. By your own admission you have ceased to be EY. In fact, you are an entirely new person who has just this moment come to be alive in a body negligently left behind with obligations by one EY, recently semantically deceased.

Seriously, what Eliezer did there was not "adapting a belief" but "pretending to have adapted a belief". His claim that, paraphrased, "I define only those people as EY" is simply false as a matter of reporting on his own brain.