Don't know how real is possibility that Democrats can make last minute change. But if the would do, no matter what they will choose, I feel that would be instant loss. It would be smaller name than Biden and also it would be showing weakness of your whole party - you don't change horses in midstream. I doubt that voters who are still not decided would accept such thing positively.
Biden or any other candidate they will choose, I think the Democrats have lost it at the debate, since the leadership under Trump administration is way far better that what Americans are facing, it quite clear that thet would rather vote the devil they know than the Angel they do not know, if the Democrats were to replace president Joe Biden.
I watched part of the debate and I asked myself, How did the "great" nation end up in such a mess. Out of millions of people across, are those the only two that have the vision to run for presidency or lead the Country?
Did you just forget about the corona virus event, it was used to get Trump out of the office and institute Joe Biden, and now that is almost four years after election, most of Americans have come to realize their mistakes by voting Joe or got disappointed by his performance.