It doesn’t change the fact he said it to begin with. Read his words. He’s not mincing them. He thought it would be a huge story, and he wanted to be one of the journalists—if not the journalist—who broke it. He underestimated the ability of Obama’s WH to lie and get away with it.
Read the transcript again. He repeated the same story all that day. Something changed by the next day. He didn’t suddenly realize he’d been repeating a figment of his imagination. He was told to alter the truth. Why would anyone suggest otherwise?
Evans was a Hollywood gossip reporter unaccustomed to having his stories closely scrutinized. If you listen to the interview where he was forced to contradict his own words, you get the sense that he suddenly realized he had stepped into it, and was just trying to back away from a claim he couldn't prove, without admitting he made it up.
If you want to believe someone made him change his story, be my guest. It would be the birther fallback position of expanding the conspiracy to explain away unwelcome news.