Do you think Oprah owes an apology to her Atheist guest for saying, "I wouldn't call you an Atheist."
It was insulting to guest. She should be sorry for offending them whether her personal opinion has changed or not. If I told someone, I wouldn't call you a Christian, and they called themselves Christian, they would expect and deserve an apology.
If she has come to understand that her thinking is wrong, yes. Otherwise no; it's her personal belief.