I'd love to say it will change the likely outcome, but I'm pretty sure no candidate has ever been subbed in this late in the race and gone on to win. History says people prefer a known quantity. Even if Harris is the new candidate, people don't pay attention to VPs, so she is kind of an unknown quantity to your average voter. AFAIK she hasn't participated in any "proper" debates with Trump

On the other hand, assuming it is Harris, the right's attacks on her are likely to be a lot more openly bigoted, which will lead to less people wanting to vote Republican. The Republicans tend to do better when the criticisms of their opponents are based on something legitimately concerning, like Biden's age. We are about to see some incredibly disgusting rhetoric from republicans, and it's liable to make them less popular. Much as I dislike all establishment neoliberal democrats, I have to say I feel sorry for Harris for what she is about to endure if she becomes the nominee. The right has gotten a lot more bold in their willingness to take the mask off since Obama was president, so if you thought the shit he had to wade through was bad, it's nothing compared to what they will fling at Harris.

The biggest thing at this point is that it's way too late to have a primary, so whoever is selected, they will be subjected to the very legitimate criticism that they were simply appointed as the nominee by the DNC. That would have basically been true even if there was a primary, but at the very least the primaries create the illusion of a democratic process. People aren't going to be psyched to vote for Biden's last minute substitute.