People fall out all the time. It doesn't mean one or the other are not truthful.
Marina gave a genuine excuse for why she lied the first time about Oswalds Mexico City trip. She didn't know exactly why Oswald went to Mexico City and so was unsure if she should admit he went there or not.
The question that should be asked is: Why do we have to rely on testimony from Marina and others to determine that Oswald was in Mexico or not?
The Russian and Cuban Embassies and Consulates in Mexico were on 24/7 surveillance by the CIA, so why is there not one photo, not one taped phone call, of Oswald in Mexico? Do we really believe that all the CIA's systems failed at the exact same time? Really?