Undecideds appear to be breaking between even and slightly for Trump. I was kind of expecting that but don't think the Democrat is in trouble until you start seeing undecideds just defaulting to Trump en masse.
Nope, look at the actual data. This is the change from the base numbers to those including "probably vote for":
Biden +7 -> +6 (Trump gains 1)
Sanders +5 -> +5 (no change)
Warren even -> +2 (Trump loses 2)
Harris -3 -> even (loses 3)
Booker -7 -> -3 (loses 4)
O'Rourke -8 -> -4 (loses 4)
Buttigieg -8 -> -6 (loses 2)
When "probably" is added, Trump picks up a point against Biden, sees no change against Sanders, and loses ground to everyone else.