Rich Hailey speculates that the Democrats know what they are doing in Rathergate: they want Kerry involved in a scandal that will let them replace him on the ticket (link from Instapundit).
One major problem with this theory is that the Democrats couldn't expect that most states would allow the change. Not all states have supreme courts like New Jersey's that think the law is theirs to make up. Well, not to that extent, anyway. Well, or at least some of them wouldn't be interested in helping the Democrats.
But here is a minor variation in the plan: the Democrats could exploit the elective system to have the same effect. They leave John Kerry on the ballot, but tell everyone that the electors will actually vote for Edwards (or Hillary, if you are willing to go that far into tinfoil-hat land).
Problem: how many states have laws requiring the electors to vote for the guy on the ballot? How many of those laws have real teeth? How many of the states with laws with real teeth have prosecutors and courts that are likely to do anything about it?
The theory has another big problem that I can't answer: why would Dan Rather defend the memos? I don't see how this serves the speculative nefarious purposes. If he were part of a plan to replace Kerry, he would already be out there saying he was had by the Kerry campaign and he wants an investigation into who knew about it.