After nearly a month of trying, I've just spoken to someone at the Pixar press department who told me that Up's storyline will be "original" and not a version of any previous work.
She did not wish to comment specifically on the story about Up being a retelling of the classic tale Don Quixote, except to say that Pixar was "not responsible" for the story. And not surprisingly, she declined to give me any new details regarding Up.
Near the end of the call, I asked her about the next WALL-E trailer, when it will be out. She apologetically told me that she wasn't sure if she could share that information with me, and directed me to the Disney press department, which I have yet to contact. But "it will be out in the spring", she said. I asked her whether that would be with Disney's College Road Trip on March 7, as I have previously speculated. She paused a bit, but again she could not confirm that for me.