Nine at the top, as a changing line, reads as a final accounting rather than a fresh test. A querent shown this line is asked to look back across the whole course already trodden, the accustomed path, the quiet solitude, the tiger's bite, the apprehensive caution, the resolute peril, and to judge whether that course, taken as a whole, has actually held complete and without failure. Great good fortune is promised, but only on that condition.
Flipping this line changes the upper trigram from Qian to Dui, Lake, giving Dui over Dui, the hexagram called The Joyous. The retrospective weighing of a course just completed becomes, on the far side of change, a hexagram built entirely from open, mutual joy, two lakes reinforcing each other rather than one trigram standing beneath another in careful, cautious relation. The movement is from vigilance finally set down to a lighter, shared contentment, as though the whole danger Treading described was only ever a passage through to this doubled ease.
As the final line of Treading's arc, this position is where the entire hexagram's meaning is weighed rather than added to, whether careful conduct near overwhelming strength has, in the end, actually succeeded. A reading falling here suggests that the matter is now complete enough to judge, and that the transformation into shared joy is available precisely because the course behind it held.