Preload doesn't seem to have any effect on when the transition from the soft to hard rate occurs because of the compression of the top out springs and because preload does not change the springs total compression once static weight is applied.
No, preload doesn't affect where the spring rate transition occurs because, with a given amount of weight load, the spring will always be compressed the same amount at rest, so the transition will always be the same distance from rest. The at rest point will vary by the amount of preload (aka change ride height).