Answer: The so-called “changes” were simply vast expansions of power by the federal oligarchy, and the two speeches were simply going from push to shove and showing his true colors.
In the first inaugural address, he says that he has no interest in interfering with slavery in slave states, and the federal government can pass any law that isn't specifically prohibited by the Constitution; and therefore only use Force to collect the federal taxes.
So basically it was purely a legal declaration.
His second inaugural address, rather, dealt with a vast expansion of power in a very Theocratic manner, claiming that the war was God's punishment to both sides for the American scene of slavery existing at all. And the Almighty will to abolish it by the wrath of God, and equating it with supposedly “preserving the nation.”
So his speeches went from plausibly denying corruption, to holy war for Empire, claiming to be doing God's will and going from president to Pope.
Consider his first inaugural speech:
Doing this I deem to be only a simple duty on my part, and I shall perform it so far as practicable unless my rightful masters, the American people, shall withhold the requisite means or in some authoritative manner direct the contrary.
Here he cleverly disguises his exercise of Power by claiming to act in the American people's name.
Then in his second inaugural address, he is far less tactical and more brazen in his attributing the entire Fiasco to the Holy Spirit, in order to show everyone that the federal government is boss:
The Almighty has His own purposes. "Woe unto the world because of offenses; for it must need to be that offenses come, but woe to that man by whom the offense cometh." If we shall suppose that American slavery is one of those offenses which, in the providence of God, must need come, but which, having continued through His appointed time, He now wills to remove and that He gives to both North and South this terrible war as the woe due to those by whom the offense came, shall we discern therein any departure from those divine attributes which the believers in a living God always ascribe to Him? Fondly do we hope, fervently do we pray, that this mighty scourge of war may speedily pass away. Yet, if God wills that it continue until all the wealth piled by the bondsman's two hundred and fifty years of unrequited toil shall be sunk, and until every drop of blood drawn with the lash shall be paid by another drawn with the sword, as was said three thousand years ago, so still it must be said "the judgments of the Lord are true and righteous altogether."
With malice toward none, with charity for all, with firmness in the right as God gives us to see the right, let us strive on to finish the work we are in, to bind up the nation's wounds, to care for him who shall have borne the battle and for his widow and his orphan, to do all which may achieve and cherish a just and lasting peace among ourselves and with all nations.
So in four short years, he went from playing politician, to Playing God.
And he proved to be but the first of many who did likewise afterward, starting with Leopold, as Lincoln's GOP Empire touched off a brewing philosophical conflict worldwide, with sophistry on all sides leaving to disaster.
Explanation: I found this from Quora. :)