I don't agree with Lichtman. He is basing his reasoning on past events in contexts that have nothing in common with today's unprecedented situation.
Today the polls give Biden and Trump about even. Those who would vote for Biden would presumably do so because they consider him a better option than the slightly crazy overgrown lout Trump. There are also many who will vote Democrat on principle.
If, instead of Biden, the Democratic candidate were one of the three "possibles," all people with a good public image, solid background, political experience and a generally positive record, why would they not choose one of them, rather than Trump?
In other words, I don't think a potential Biden-voter would switch to Trump because instead of Biden the candidate is Harris or one of the other two.