They don't. They leave Kerry's name on the ballot. So ...
What the DNC won't be able to easily do, and they may not care, is change the name of the presidential candidate on the voters ballot in each and every state. But naming the presidential candidate on the voters ballot is merely a convenience for voters. When the voter pulls a party lever, the voter's vote actually goes to an elector. Electors and their alternates have already been timely named by their respective parties.
As a practical matter, the DEM party wants to come up with a scenario that results in obtaining a majority of the electors' votes. But there is no legal impediment to changing the candidate -- even after the November election (it would be bizarre, but it is legal and possible).
So do YOU think Hillary will run? Or who would take Kerry's place? (Edwards is too inexperienced, I don't think the Dem's would pick him). What about Dean or Lieberman?