They wouldn't have to convince millions of Americans to vote for the new candidate, just a few hundred Democrat faithful who would be easy to threaten, cajole, and/or bribe.
What the Dems would have to do is convince the voters to vote for Hillary not necessarily knowing who would be swapped for her. This is the interesting choice: whether to specify someone as a replacement before or after the election. Either alternative has advantages and disadvantages.
Still, I think any uncertainty, would tend to take some votes away from Hillary. She might get some Dems to get off their couches to make a sympathy vote, but they could lose many more votes to those Bernie voters who would feel totally burned by a system that propped up an unhealthy candidate that couldn't even make it through the summer.
The other aspect to this is the write-in factor.
More than a few 'Rat voters may write-in their POTUS/VPOTUS candidate, knowing that Canky was at or near room temperature.
Those write-in votes would NOT be tied to 'Rat electors.