Evelyn Lincoln,
Kennedy and Johnson (1968) "As Mr. Kennedy sat in the rocker in my office, his head resting on its back he placed his left leg across his right knee. He rocked slightly as he talked. In a slow pensive voice he said to me, 'You know if I am re-elected in sixty-four, I am going to spend more and more time toward making government service an honorable career. I would like to tailor the executive and legislative branches of government so that they can keep up with the tremendous strides and progress being made in other fields.' 'I am going to advocate changing some of the outmoded rules and regulations in the Congress, such as the seniority rule. To do this I will need as a running mate in sixty-four a man who believes as I do.' Mrs. Lincoln went on to write "I was fascinated by this conversation and wrote it down verbatim in my diary. Now I asked, 'Who is your choice as a running-mate?' 'He looked straight ahead, and without hesitating he replied, 'at this time I am thinking about Governor Terry Sanford of North Carolina. But it will not be Lyndon.'"
http://spartacus-educational.com/JFKsanford.htm I have not seen any evidence frokm Mrs. Lincoln or anyone else that JFK mentioned Smathers in this connection.
As I said, I seriously doubt that JFK intended to drop LBJ from the ticket. But if he did, Sanford would make a lot more sense than Smathers. You want to drop LBJ because he is too much of wheeler-dealer, a man with ethical problems--and you replace him with George Smathers?! Not to mention that JFK had committed himself to the civil rights bill, and was unlikely to choose an opponent of the bill (and a signatory of the Southern Manifesto) as his running mate. And If JFK was going to lead a campaign against the seniority rule, Smathers would be the last man he would want.