One opinion in the Talmud (Megillah 13a) tells us that the “changes” mentioned here refer to Heigai giving Esther “Jewish food,” which presumably means kosher food.
Another opinion is that he gave her pork bacon.
A third opinion, that of Rabbi Yochanan, says Heigai fed Esther seeds. His proof is that, in regard to Daniel and his comrades, the verse (Daniel 1:16) says “The cook took away the bread, and also gave them to eat seeds.”
If she did eat kosher food, Ben Ish Chai in Ben Yehoyadah wants to know why this did not give away her Jewish identity. He answers that, first of all, everybody knew she was from Mordechai’s house (as we mentioned in a previous blog), so it was possible she became used to kosher food in his house even if she were not Jewish. He also says that kosher food has a reputation for being more healthy than non-kosher food (http://www.npr.org/templates/story/story.php?storyId=129649433), anyway, and it was safe to assume Esther may have preferred it for that reason. Perhaps this last idea is alluded to in the Talmud’s use of the phrase “Jewish food” rather than “kosher food.” In other words, he gave her food to eat the way a Jew is supposed to eat, primarily for health reasons and nothing more (Mishnah Torah, Hilchos Deyos 4:1).
Actually, regarding seeds, there is a custom (Shulchan Aruch Orach Chaim 695:2, Mishnah Berurah ibid., sub-paragraph 11) on the night of Purim to eat edible seeds (sunflower seeds, etc.) because of this verse. What is interesting is that the custom is to eat the seeds specifically at night. Rabbi Moshe Meir Weiss suggests that we do this at night because Esther was eating seeds in an attempt to hide her identity while simultaneously abiding by Jewish dietary laws. We, too, follow suit by eating the seeds at night, a time of secrecy.