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.
The Talmud (Megillah 5b) explains each of the different expressions for this holiday to mean a different method for celebrating the day. Simcha (“joy”) is interpreted as not giving eulogies (in the event of a death); v’mishteh (“and feasting”) is interpreted as prohibiting fasting; and v’yom tov (“and the holiday”) is interpreted as prohibiting work on Purim. Later, the Talmud (Megillah 7a) interprets the phrase mishloach manos (“sending gifts”) as the requirement to send through a messenger at least two kinds of food to at least one friend.
The Rambam (Mishneh Torah, Hilchos Megillah 2:15) writes that even two poor people are required to send another poor person some food to fulfill their obligations.
The Trumas HaDeshen writes that the exchange of food is intended to make sure everyone has enough for the feast.
M’nos HaLevi writes that we send gifts to promote friendship because achdus (“unity”) rescued the Jews.
The Midrash HaGadol on Devarim points out that this demonstrates the greatness of chesed because we were rescued because of it.
Ginzei HaMelech writes that we use a messenger because this shows achdus (“unity”) in requiring another person to get involved in this mitzva. Similarly, he points out, this is why Megillas Esther always uses Yehudim for Jews, since the root of that word is echad, one. Furthermore, the giving of gifts through messengers acts as an additional tikkun for Yaakov’s giving gifts (Bireishis 32:14-17) to Eisav, the ancestor of Amalek, through messengers.
The Vilna Gaon and Midrash Shmuel note that the Jews’ celebrating in this way parallels the three parts of Haman’s plan (Esther 3:13); the joy serves to counteract Haman’s plan to destroy the Jews, the feasting serves to counteract Haman’s plan to kill the Jews, and the Yom Tov serves to counteract Haman’s plan to annihilate the Jews.
The Vilna Gaon writes that, eventually, Purim was not accepted as a full Yom Tov because that would keep people from performing the other mitzvos of Purim.