While reading about history of Saturn family of rockets, I read that all the Saturn rockets were initially designed to meet defense requirements for heavy lift vehicles. I was reading a NASA publication: https://history.nasa.gov/MHR-5/contents.htm with the intention to find out at which point did the objective of "creating heavy lift vehicles for Military use" changed to "launch vehicle for the Apollo"... Can someone lead me towards "When & How" of the same?
It's not possible to indicate a single point in time for this change; it was more of a process that started in 1958 with the inception of NASA. The Advanced Ballistic Missile Agency (ABMA), and with it the team of von Braun, was transferred from the Department of Defense (DoD) to the civilian NASA and with that transfer Saturn development was out of control of the DoD. From Stages to Saturn, chapter 2:
But the days of Saturn's ties to ARPA were numbered. After letting the issue simmer on a back burner most of the year, York raised the transfer issue again in the autumn of 1959, and this time got the support of both the Secretary of Defense and President Eisenhower. Given the inclinations of the NASA hierarchy, ABMA's transfer from ARPA became inevitable. NASA's own requirements for a booster the size of the Saturn had been made more explicit as a result of the Research Steering Committee on Manned Space Flight, chaired by Harry J. Goett of NASA's Ames Research Center. The Goett committee, formed in the spring, had considered NASA goals beyond the Mercury program, and during the summer a circumlunar mission emerged as the principal item in NASA's long-range planning. A manned lunar landing required a much larger booster-Saturn. With potential mission and booster requirements finally outlined, satisfying Glennan's criteria to have a specific mission for the launch vehicle, total NASA responsibility for Saturn was obviously needed.
The Dyna-Soar program (Air Force) sputtered on for a bit longer, but in 1961 it became evident that NASA just did not have the manpower to handle all the various configurations and mission profiles in parallel. Eventually a manned-lunar mission became the sole focus for NASA and hence the military applications for Saturn were no longer considered. From Stages to Saturn, chapter 3:
During 1961, configurations seemed to change month by month. In January, the C-1 vehicle changed from a three-stage to a two-stage booster, eliminating the S-V upper stage to leave only S-I and S-IV stages; but S-V development continued during February. By May, the C-1 had become a possible three-stage vehicle again, including Block I and Block II interim versions. In February, the C-2 was ticketed as a three-stage vehicle for Earth-escape missions (featuring an S-II second stage); in May, there was talk of a need for an even more powerful vehicle for circumlunar missions; in June the C-2 was dropped in favor of a C-3, although Nova would continue; later in the year, there were plans for a C-4, along with a solid-booster C-1. By the end of the year, there was also the C-5. One result of this was the decline of Dyna-Soar, whose position as a NASA payload essentially evaporated after the C-2 cancellation in June.