I did not know Cenk used to be a Republican. I do know he was a die-hard Democrat since the Bush years. He started his "Young Turks" broadcast in 2002.
Maybe he was Republican pre-Bush, before the GOP became completely co-opted by Neoconservatives and Moral Majority-types?
Cenk is very active in pushing "cool" and "hip" Democrats like Barack Obama (he was huge in the "Change", "Hope", and "Yes We Can!" scene), and Bernie Sanders. Though I don't recall him being as active with The 'Bern than he was with Obama. Admittedly, I stopped paying attention to people like Cenk, Bill Maher, etc sometime in early 2010s.