There is evidence - strong evidence - that you are right. Many in the North did not care what the Constitution said. They had the bayonets to overthrow the pro-slavery U. S. Constitution and - according to this version of history - they did fight to overthrow it. Or at least substitute a synthetic Constitution.
Famed Lincoln biographer Garry Wills writes it went down like this: “Lincoln, at Gettysburg, performed one of the most daring acts of open-air sleight-of-hand ever witnessed by the unsuspecting. Everyone in that vast throng of thousands was having his or her intellectual pocket picked. The crowd departed with a new thing in its ideological luggage, that new constitution Lincoln had substituted for the one they brought there with them. They walked off, from those curving graves on the hillside, under a changed sky, into a different America. Lincoln had revolutionized the Revolution, giving people a new past to live with that would change their future indefinitely.”
So, in your view it would only be a real Constitution if it allowed and supported slavery.