When people run for President (and only when people run for President), the candidate is not the "candidate" on the ballot. The pres. candidates' names appear on the ballot, but the actual votes are cast for delegates to the conventions in the primaries, and Presidential Electors in the general election.
So, swapping support may be difficult for ordinary candidates, but it is relatively easy and simple when running for President.
John / Billybob
Do you really think the average Democratic primary voter can comprehend the "A vote for Clark is a vote for Hillary!" scenario? I don't. They'll vote for Clark and will wonder why Clark isn't president.