On my own experience, in my four times taking CX883 (the earlier flight), I've gotten upgraded the same four times (was either Silver or lowly GR during the time of these flights) to J. These also occurred mid year (JUN-AUG) and end of year (NOV-JAN) periods.

You are also correct to note that a lot of 881 passengers were offered to 883, so your theory is also logical and plausable but I do get upgraded nevertheless.

I've taken a few 881s too but i seem to recall a 50/50 chance of op-up.

The only time i never got to upgrade from Y to J was when i took the afternoon 885 flight, but me and almost everybody in the cabin have our own rows.

And in my experience with the four flights back from HKG to LAX, I've never gotten chance to op-up to J, the first few times noticing that J was indeed full, while the latest flight, i notice that there were a few op-ups but they were for diamond (noting the baggage tags that comes out of carousel).