Sisko goes into the past, takes the place of Bell, returns to the future, and it's his face, not Bell's, that one sees when reading this man's biography. Yet, the future he returns to is the one we know. Changes in timeline don't have to be major; they can be as small as a change in image.
And it could also be argued that Sisko was always Bell.
We did see a 'real' Bell back in 2024, but that was it. In the 24th century, nobody checked Bell's records and saw that version. The only time anyone did that, it was after all the time travelling was over, somebody checked the image of Bell and saw Sisko. So how do we know that Sisko wasn't always supposed to be Bell? Another predestination paradox.