From that point, until the actual election, a switheroo would require either getting a sufficient number of states to print a new ballots, or getting voters to vote for, say, Kerry, when they really meant Hitlary.
Then after the election, a switheroo would require getting the winning Electoral College delegates to decide on someone other than whom they were sworn to support (say due to an unexpected death).
Though of course, you are right, prior to the Convention results being taken up by the states to print ballots, it's up to the political party.