[GreatOne]

It's my understanding that the route was published in the paper that day, which did infuriate the Secret Service, and was a cause for minor route adjustments.

[Bloody Sam Roberts]

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'.