The wiki at the top of each page in this thread, albeit brief, sheds some light on the reasons and the outcomes. And whomever posted this is spot on: "Generally, this is good news for those on the upgrade list and a possible chance of Op-ups for some." Indeed, based on the flight metrics at the moment, everyone on the UG list will clear. If Y was full (from a physical capacity standpoint), they would them op-up some folks to J. Not the case here, because of capacity limits. Thanks runner450 for clarifying that it's 75% now (I knew it was a certain percent, but didn't say as I didn't know, and news articles were mixed on the current capacity % of any given aircraft).

As for holding back upgrades; It's always been a 'thing' but especially on flights with such limited capacity--I think China is among the most tightly restricted in the world right now. They should/could probably clear a dozen or two for the 12/24 flight, but that's not their thing right now--probably 8 or 4 hours before departure, and perhaps just at the gate. Maybe it's been demonstrated that X amount of people, on average by J class tix a day or two before. I don't know specifically why, but look for this to continue especially to China--mostly because of the very limited capacity to China, not just on UA but across all airlines (I believe the current 75% capacity limit even applies to China-flagged carriers). However, I doubt this is a scenario where they don't clear people in advance to keep them out of lounges--but I wouldn't put past them the more I think about it.

As for your final question, with almost certainty, they will clear everyone before departure. Not to scare you, but with UA, anything can happen!