No you don't understand, you know whow Han talks about the events of Ord Mantell inthe beginning. All what happened there, that wouldn't have been Han's anymore. It wouldn't be like, Dash is there, he is new! No, Leia would have known him for a long time, just it would not have been present on screen. That's how also the love interest began with Han and Leia, something was already between them but that began off screen, between A New Hope and Empire. That wouldn't be different if it was Dash and Leia. Also, we're not really talking about replacements, cause how you describe Han as a rogue is very simple. Dash would be different of course, he is different. So that would make him more unique instead of just a replacement.