Did Rav Ashi Deny Knowing Testimony in Beis Din?

There is a known rule throughout Shas that once a person has finished testifying, he cannot go back and alter his testimony. In our Gemora (Kesuvos 20), Rav Ashi denied knowing testimony for Rav Kahana, but later remembered and testified? How could he do so? Didn’t he already testify that he didn’t know? The Hagaos Ashri (#16) states that it is therefore clear that he did not deny knowing testimony in Beis Din. Other Rishonim, however, argue that the rule above is only if someone actually testified regarding events that happened. However, if he says he is not aware of the events and later remembers, this is not called going back and changing what he had said. He simply remembered that he knew testimony.