But unless I'm misunderstanding something, the change in approval is already factored into the current predictions. If there is a greater than expected turnout

But unless I'm misunderstanding something, the change in approval is already factored into the current predictions. If there is a greater than expected turnout by a group of people voting - in aggregate - more for Biden than for Trump, then that favors Biden relative to the current predictions. So that's not going to be a reason for a surprise in the other direction. There are plenty of reasons our predictions might not match reality, but they're not going to be wrong in that direction for that reason.