That's interesting. It is possible then that some a-hole AM radio beanhead may have picked up on this as a minor news story and broadcast the changes. There were also other people lined up on the new motorcade route through Dealey Plaza? Perhaps they got wind of the change the same way Oswald did.
Although, I'd still like to know the definition of 'last minute'.
I hate to say this, but I can't answer your question. I've read over 25 books on the assassination, but it's been about 5 years since I've re-read any of them. I do remember the route being published in the paper, and I remember that the route was changed slightly because of it. But staring at the obvious, I cannot give you an explanation as to why people were lined up in Dealey Plaza when it wasn't in the paper that morning. I can only think that people were told by cops or other people in the know where he'd be going.
Hate to not give you a satisfactory answer, but that's all my forgetful memory can recapture.