LeonS, I'm sorry to hear about your experience. It stinks, and it's not fair.
That said, this is, to some degree, par for the course at United. Your experience really divides into two parts: (1) you got downgraded; (2) you had a very bad customer service experience. I think the most important is #2. Getting downgraded... well, stuff happens, on any airline, occasionally. Somebody had to get inconvenienced.
The bad experience with the manager and the lack of an advance call, though... yeah. It's not necessarily common, but this is an area ("service recovery") where United has been pretty bad at least since the merger. If you continue to fly United, expect that this can happen. It's unlikely to happen to you again soon, but on the occasions when the fecal matter hits the fan, you should not rely on United to treat you nicely, gently, or reasonably. We had a saying around here for a while: "Flying United is not for amateurs." You gotta know the rules, be prepared to fight, have a thick skin, and always be prepared. Otherwise, try to book non-US carriers (or Delta as a last resort).
Now, about the downgrade. You asked how this could happen. Obviously I don't know the details, but it sounds like after the early flight canceled, they were trying to cram 2 airplanes worth of passengers onto one flight. Even if you were booked in full fare J, there were two categories of passengers with higher priority:
1. Any GlobalFirst fares.
2. Any Global Services (top tier elite) passengers with any business class fare.
If the earlier flight had a GlobalFirst cabin, then it's entirely possible that there were 20-30 passengers getting pushed onto your flight who had higher priority. And although in normal circumstances passengers with confirmed seat assignments don't get trumped by rebookings, this particular situation (an entire international flight goes MX) is not normal circumstances. So it's entirely possible that 20+ passengers on your flight got bumped out of biz, including all upgraders, all discount J fares, and you.
It's still not fair or right, but that's how it may have happened.