Haman's original decree, signed by the king, was that the Jews of Persia would be killed.
How did the Jews react? With resignation. They cried. They fasted. They sent Esther to intervene. But, from the text it seems that, worse comes to worse, they were content to follow the law. We see no indication, from the text, that the Jews of Persia planned to use force to resist the decree.
The second decree, the one published by Mordechai and Esther, gave the Jews permission to defend themselves. The reaction? Joy, mirth and celebration! L'yehudim hoyita simcha, v'soson! V'yehefach may'avel l'yom tov!
What changed? Only the law of the land.