Generally, state laws require that candidates on the general election ballot be formally nominated by their political party’s national convention held in a presidential election year. The laws vary by state.
So theoretically, the Dems could re-open their national convention and have the delegates nominate a replacement. Whether or not their party rules allow for such a process is another story. There could also be state laws that require the nominee’s name to have been on a primary ballot or to have been selected by party caucus.
Another limitation would be state deadlines for political parties to submit the names of their nominees to the SOS for placement on the general election ballots. Typically, it’s at least 60 days before the general election. Sometimes more.
They will just use the Lautenberg exemption.
Laws and rules have never hampered Democrats.