Second, redefining the F function will update its association (or binding) to the F symbol, but the previous function will still be available if it has been referenced somewhere before the update. For example:

(setf (symbol-function 'foo) #'1+)
(let ((old-foo #'foo))
  (setf (symbol-function 'foo) #'1-)
  (funcall old-foo 42))

It still return 41 on sbcl