R’ Dovid Feinstein writes that the expressions are written in a different order than the previous verse (Esther 9:18) because the original celebration was spontaneous, and not following any specific rules. Mordechai would later (Esther 9:21) establish Purim for future generations with changes.
Yosef Lekach notes that everything mentioned in the verse needs to be artificially “made.” In that first year, happiness was a natural, organic reaction. In the future, it would have to be manufactured artificially.
Malbim writes that the Jews did not feel the need to celebrate the first year because they didn’t know the decree and thought that their victory was due to the king’s decree.
R’ Yehonason Eibshutz and the Chasam Sofer note that in the first year, the Jews accepted Purim as a Yom Tov, so Mordechai expected them to feed the poor.
After all, the Rambam (Mishneh Torah, Hilchos Yom Tov 6:17) writes that people have the responsibility of feeding the poor on a Yom Tov. Later, when the people would not see Purim as a Yom Tov, the order was switched around in order for the people to still feel responsible for feeding the poor.