To fundamentally change something means that, since you despise and do not respect what that something is, you have to destroy the building block(s) that is securing the existence (and sustainability) of that something you want to 'fundamentally' change.
In Barry's case, its the United States of America (and 'we the people') that he despises and does not respect.
And so his undertaking therefore, is to utterly destroy the nation's founding documents (i.e., the US Constition and the Declaration of Independence).
No matter what one thinks about Barry, no one can deny that he has done a truly masterful job at what he has (all but) succeeded at accomplishing thus.
Should he manage to con his way back again, there is no doubt that