First, we must acknowledge that it is not clear that the Declaration of Independence itself changed American conceptions of freedom. The conceptions embodied in the Declaration must have existed in the colonies already since the signers of the Declaration were willing to agree with them.
However, if we must argue that the Declaration changed American conceptions of freedom we can argue that it did so by making freedom a broader concept that applied to more people. Before the Declaration, we can argue, liberty was seen less expansively. People did not believe as much in the idea that citizens should be free from government interference in all aspects of their lives. We can also say that American colonists were less committed to the idea that “all men” were equal to one another. From this point of view, the Declaration explicitly extended rights to all men instead of having some men be seen as superior to others.
By expanding the definition of rights, and by extending freedoms to more people, the Declaration of Independence can be said to have permanently changed the American definition of freedom.