I can't address directly the equipment change portion of your question, but I'm sure others here will be able to....
OTOH, I've had ticketed, upgraded segments on my "active itineraries" appear and dissapear regularly over the past 60-90 days. I made a few phone calls when it first started to happen, only to be told that everything was fine.
I've flown 2 of these flights without any incident whatever, so IMHO, I'd guess that your situation is more ual.com related than "seat change" related. Here's hoping that I'm right!!
Best wishes.