I don't know about a change of heart, but the simplest reason for Kim to tell the truth is what the AV said about her.
It's the only way out for her. She knows the Fonger is wounded and the jackals are circling. She's got enough street smarts to figure that out...
Well, Mangum wasn't raped, so Kim didn't help with a rape. If someone said I helped with a rape, it would piss me off big time. I'd be ballistic, in fact.
The simple fact that Mangum said this and Kim wasn't indicted for aiding the felonies the boys have been charged with says a lot about what Nifong really believes - which is that he, too, knows there was no rape.