Per wikipedia, the democrats have 4 options for their plan B.
John Carney, U.S. Representative, former Lieutenant Governor of Delaware and candidate for Governor in 2008[4]
Thomas P. Gordon, County Executive of New Castle County[5]
John Kowalko, State Representative[6]
Peter Schwartzkopf, Speaker of the Delaware House of Representatives[6]
Rating for this race stays at Safe D for now.
There's always a possibility that someone who had previously ruled this out because they knew Biden would run would now change their mind.
But I expect no one will announce anything immediately because that would be bad form.