So basically you're confirming the fact (often disputed here quite vigorously by certain posters, with a reference to a rather vaguely worded post from an AC rep about 'upgrade' priorities) that Aeroplan tickets with at least one segment confirmed in J, are basically at the top of the waitlist (and superior to the 'upgrade' list) for travelling in J for the remaining segments. And this is what you took advantage of, because you were able to find inventory in the Y cabin at some point in the day (a significant likelihood on most lighter travel days on such a busy route!).

Is my assessment of the situation correct?