Why do the Sages actually want to change an accepted Jewish law? One that gained authority and spread among all the different groups.

???

On the thirteenth day of the month of Adar, and they rested on the fourteenth day, and made it a day of feasting and joy. But the Jews in Shushan assembled on the thirteenth day and on the fourteenth day of it, and rested on the fifteenth day, and made it a day of feasting and joy. Therefore the rural Jews, who live in unwalled towns, make the fourteenth day of the month of Adar a day of joy and feasting and celebration, and of sending portions each man to his fellow. And Mordecai wrote these things and sent letters to all the Jews in all the provinces of King Ahasuerus, both near and far, to establish for them that they should observe the fourteenth day of the month of Adar, and the fifteenth day of it, every year and year, as the days on which the Jews had rest from their enemies, and the month that was turned for them from sorrow to joy and from mourning to a holiday; to make them days of feasting and joy, and of sending portions each man to his fellow, and gifts to the poor. And the Jews accepted what they had begun to do, and that which Mordecai had written to them.

There is no mention of any custom to fast at all, so why bring in a sense of upheaval? It is the opposite of what should be.

Suppose a person had a terrible illness that could have killed him at any moment, and he fasted and afflicted himself and the Holy One, blessed be He, healed him. Would he then go and make that day on which he fasted into a fixed annual fast? In your view, yes?
B. When did the Sages decide that this whole idea of upheaval was necessary?
The very reading of the Megillah brings one to remember the great and tremendous providence that God sent to His people Israel.

Rabbi, are these Jews only from the seed of Judah or from all the tribes? Because nowadays everyone observes the days of Purim without exception.

And if there had been a mention of fasting in the Megillah, then too you would ask why the sages of that generation (the generation of the Megillah itself) were changing Jewish law. In what way are they superior to the sages of later generations? That is what they saw fit to do, and I do not see anything wrong with it. It is possible they saw that reading the Megillah was not working properly and thought it better to convey to us the upheaval of that time.

It does not work that way. In order to convey upheaval or dread, there has to be a real threat. In the Megillah the real threat was over and done with, and that is the point of the Megillah: to rejoice in our lives and in the fact that God saved us despite the evil fate (in the view of the nations) that was supposed to come upon us because of the lot that determined the day of our destruction. Despite all the plots, the salvation came; even without Esther, the salvation would have come from somewhere else because there is providence. That is what Mordecai believed.
Good thing you did not say that we should go destroy our enemies in these days. Aren’t you going too far, Rabbi, when you argued against me by asking in what way they are superior to the sages of later generations? So the fast has no meaning, it does not arouse any upheaval at all, and the reading of the Megillah is not affected by the fast.
Apparently everyone will remain in his own position.
In every generation they rise against us to destroy us, and the Holy One, blessed be He, saves us from their hands.

Each generation and its sages, each generation and its interpreters — Mordecai alone decided for everyone to accept these days of Purim every single year.