I don't think Andrew Sullivan has ever been considered a "conservative". More of a liberal who knew that his parties stance on total and complete obstruction to reforming social security was the wrong stance and he encouraged young people, based solely on the issue of privatizing social security to vote for Bush.
He was an early and vocal supporter of the decision to invade Iraq and was scathing in his criticism of the anti-war movement. But, bizarrely, the moment Bush announced his opposition to gay marriage Sullivan did a 180 on Iraq. He tied himself in rhetorical knots to explain why he dedided to support Kerry, a man for whom he had previously had nothing but contempt.
Sullivan was and is a conservative Catholic, but is so pro-gay marriage it throws him off balance. I saw him on a panel one time where the other gays wished him dead. I don't understand his flip-flop on the war, but I disagree it's because GW is against gay marriage.