I noticed United starting sending emails notifying of a switch into the GF cabin. The headline is "We've moved you to a better seat for your flight to xxxx".

This week I received that email and got a prime 2K seat, and then noticed a day later that I was switched to an old C class sardine seat. As far as I'm aware, there was never a swap back and forth between aircraft that would justify reshuffling to any degree let alone a downgrade, but perhaps. I called the GS line and asked about the seat change and what they could do to get me back into GF and it turned into an awkward/tense discussion where no movement was made on their end...
"Sir, the same service is provided throughout both cabins."
"Yes, I know that, but the seat is much better in the former GF cabin"
"Sir, what is considered a 'better' seat is very much a subjective matter"
"Oh come on, the email from United literally says 'we've moved you to a better seat'. I think we all know what is the better seat here."

She had no clue this email was being sent by United. When I forwarded this evidence of my prior seat selection to the GS email address, they said there was nothing they could do about future reservations, only past customer service issues. I said, well now it's a customer service issue because I've been downgraded. Silence. Ugh. GS service is going downhill.

In your flight to Shanghai