Only in Britain does Anglicanism date back to the Reformation.
The Church of England first separated itself from the Roman Catholic Church in the reign of Henry VIII. Henry did not turn it into a Protestant church - that did not begin until the reign of his son Edward VI - but he did make himself sole head of the English Church, thereby establishing that it did not owe allegiance to any international body. According to Article 37 of the Thirty-nine Articles, 'The Bishop of Rome hath no jurisdiction in this realm of England'. If we do not grant jurisdiction to him, why should we grant it to archbishops in other parts of the world? The Covenant would in effect provide them with power to instruct us, just as it is designed to provide them with power to instruct the churches of the USA and Canada.
However the main reason why it would be the biggest change is that it would turn Anglicanism into a confessional church where church members are expected to believe what they are told. Covenant supporters have increasingly claimed that there is one permitted view and it is un-Anglican to dissent from it.