It was not changed. It was rejected and replaced:
Canon XXIX.
Christians must not judaize by resting on the Sabbath, but must work on that day, rather honouring the Lord’s Day; and, if they can, resting then as Christians. But if any shall be found to be judaizers, let them be anathema from Christ.
It is not difficult to recognize the precursor to the most virulent antisemitism in this rejection.
This obviously false claim that represents that Sunday worship did not exist until sometime in the 3rd or 4th century does not have coherence with history actual .
There are other historical writings other than the New Testament such as....
Diadche -- between 80 and 140 AD, Ignatius, about 107 AD, Aristides -- 125 AD, Justin Matyr -- 160 AD. And they all record similar writings , as as this......
Victorinus, about 280 AD, "And let this become a rigorous fast, lest we should appear to observe any Sabbath with the Jews. For concerning {their Sabbath**, Christ himself, the Lord of the Sabbath, says by His prophets that "His soul hates." In His body, He abolished this Sabbath."
Aristides says much the same,"However, {the Jews** too have erred form true knowledge, in the their imagination, they think that it is God whom they serve. Actually, by their type of worship they render their service to the angels and not to God. For example, they do so when they celebrate Sabbaths."
Thus, indeed it is a very simple thing to prove that a council held in between 363-364 AD could have not instituted the practice of Christians holding worship services on Sunday in honor of the Christ's resurrection.
But the most obvious recording concerning the change of covenants and the observations of Sabbaths, comes form the scriptures of the Old Testament -- Jer. 31: 31-36. The prophet foretold of the changing covenants and how the physical nation of Israel would be taken from the sight of God when this change came about, (VS. 36), "If these ordinances depart from before me, saith the Lord, then the seed of Israel shall cease from being a nation before me forever." And indeed the physical nation of Israel was wiped from the fact of the earth by the Roman Empire, just as the prophet said, and just as the Christ prophesied -- Mathew 24: 4-35. But, there is not scripture of prophecies to be found in either book that declares that Israel will disappear from the face of the earth for 2000 years and then reappear with no tribal ID, no priesthood to council the God appointed monarch, in fact no monarch, yet we are supposed to believe this is the Israel of the Bible. RW BLU