In fairness, we should point out that it is pretty rare for this upgrade mechanism to occur, even in scenarios where it's possible to do so. While technically available, the Y/B fare restriction means that this is not often utilized. If upgrade space is available, generally discount business class is also available for sale, and the cost of Z/P (or equivalent) is frequently the same or less than a full-fare Y/B economy. Anyone buying tickets far in advance is probably not buying Y/B fares on Int'l carriers, so the scenario where this works out to the traveler's advantage is pretty rare.

But as others have pointed out, this is all academic, because you can't do it on EI regardless!