Falcon 9 was designed from the beginning with safety factors consistent with human-rating, rather than the lower safety factors required for cargo (1.4 vs 1.2 if I recall correctly). And with the very first cargo Dragon flight in 2010, SpaceX talked about how the windows and life support system pointed towards their ambition of flying humans on the spacecraft.

Now, as always happens, actually completing the human-rating of Dragon required more work than expected, and Dragon 2 is quite different from Dragon. But, as far as I know, Falcon 9 did not require nearly as many upgrades to be human-rated.