Esther is initially called Hadassah (Esther 2:7). Why does she change her name, ostensibly even before she is brought to the king's harem?
It appears that the Tannaim Reb Meir and Reb Yehudah (and others) dealt with this question in Megilla 13a.
Rabbi Meir said: her name was Esther; why was she called Hadassah?
Because of the Righteos Tzaddikim who are called Haddasim.
Rabbi Yehuda said: her name was Hadassah; why was she called Esther?
Because she hid her words (the information about her family).
See the Gemara there for more views.
Hadassah it's a name with 11 variant depending where you are in the world: Astera, Asti, Astra, Estella, Esther, Etoile, Hadassah( Bride, star, Queen), Hester, Stella. She knew her destiny, She was not supposed to pass there as "slave Jewish girl- Hadassah". The covenant of God with them in that time demanded that Her name be translated in Greek in order for her to be selected at the contest so the uncle took the name Haddassah (in Hebrew) change it to Esther (In Greek). But it is the same and one name.