While it may be possible to replace Hillary with anyone, it would be difficult to do. Getting someone else’s name on 50 state ballots at this point could be impossible. Hillary / Kaine electors in the Electoral College could be convinced to vote for someone other than Hillary, but that would be unprecedented. If Hillary drops out, Kaine may be the only choice
Someone else’s name does not need to be on the ballot, because the vote is actually for a slate of electors. Also, the ballot deadline for most states is for independent candidates, not the candidates from recognized political parties, who have a different set of rules.