I guess nothing will happen or change in 2020, because the first years of a new calendar decade are always the peak or fading out of the previous cultural decade like since WW2.
You are correct, the first "zero year" of a decade is always in truth the finale of the previous decade.
So 2020 will be the "grand finale" of "the 2010s" as a cultural moment (which I would say didn't even fully come into it's own until 2014)
But expect things to start changing fast in 2021, just like they did in 2011, sure, it won't be an overnight night, but the process will start.
2020s-2030s will be very pivotal years for this century. Considering how the World Wars were the defining moments of the first half of the last century I do wonder if we will have such massive worldwide...events happen again anytime soon.
Politically I think the world is going to go insane trying to find the path forward. Optimistically though I don't think it will lead to too much bloodshed.
Worst case scenario I see another World War as being more like a Global Civil War, with various insurgencies in the various parts of the world fighting it out for the future, think not unlike what's been going on in places like Syria, just on a global scale.
It would probably be violence on a scale the human race has never before seen, making World Wars 1 and 2 look like child's play, Apocalyptic scale stuff, the entire world would become third world.
But hopefully that can be avoided.