If you hadn't become queen, the Megillah couldn't have called you אסתר המלכה. Thing is, the Megillah was certainly written by Hashem long before the Purim story happened. Because if Purim is even more important than holidays from the Torah (we see this from the fact that all others will disappear when Mashiach comes (Megilla 10b (h/t @Fred))), and the Torah was written 974 generations before the Earth was created (Avot D'Rabi Natan 31:3), all the more so the Megillah was written long before Creation. And if the Megillah would be falsified by you not being queen, cha"v the Torah would also be, as we see that the Megillah which prescribes Purim (which is eternal) might be more important than the Book which prescribes the (temporary) holidays.