It doesn't appear to me that her story ever changed. The media used their usual sloppy wording and confused the issue. As far as I can see, from the beginning it was said she went to the room after she had clocked out. The first story said "room service" (with the confusion about it technically being closed and Bryant got the exception). Then the "room service" aspect was debunked, but Cosby now reports it was "hotel related" errand. As far as I can see the girl isn't saying anything to the media, so it was the media screwing up the terms and causing confusion.
I don't know anybody saying he's guilty of rape, but the police are saying they have evidence of assault and that is why I wouldn't discard the idea he could have done it. Also, I pointed out my logic in the timeline leading me to believe he wasn't forthcoming as some thought. They said he told the police right away he'd been with the girl. Yes, changing stories do go to credibility. The girl's story has not been made in the public record to enable us to judge it.