It's too late for Hitlery to get on the ballot in most states, the Rats have been trying like crazy to keep Nader off and he filed a long time ago so their lawyers would have to do a 180. Don't think a write in is in the cards.
The DNC can change candidate at any time, up to the date the electors (not us, voters) cast their ballots in December.
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).
See http://www4.law.cornell.edu/uscode/3/ch1.html <-- 3 USC 1
US Code, Title 3 CHAPTER 1 - PRESIDENTIAL ELECTIONS AND VACANCIES
The DEM party has been fighing against Nader's PARTY, not just Nader the candidate.