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HELLO UNITED PEOPLE! LONG TIME NO SEE! TELL ME YOU'VE MISSED ME!
TELL ME!!!!!!!!!!!!!!!!1!!!!! 
It's been some time since I posted here, as I left United ~10 years or so ago, for a whole host of reasons, most of which I've forgotten by now...
Well, I've had cause to book a ticket on a UA 777 flying IAD-SFO-IAD with the old three-class configuration. It was a paid ticket (money, not points/miles) and I haven't had status on UA since 2008, after eight years as a 1K.
I was able to book us into seats 2A and 2C out the outbound and the return, in the former first cabin. Yay?
However, when I checked the reservation for something earlier this week, I noticed that United moved us out of 2A and 2C on the return and assigned us to 9A and 9B (in the narrow business cabin).
When I called United about this, they said that someone who had Premier status bumped us out of 2A and 2C, which I thought was strange, as in my experience (not on United, but on American and Delta where I've had top tier status after leaving United), an assigned seat was an assigned seat, unless there was a medical need to move someone, or an equipment swap, etc. I've never heard of or seen an airline moving someone out of an assigned seat for an upgrade.
So, the question:
Is moving someone out of an assigned seat for an upgrade really a thing at UA?
Since I started participating on FT back in 1999 or so, I was pretty good about understanding a lot of the arcane rules around fare classes, rules, upgrades, etc., but I'm 20 years older and, well, I've stopped paying attention to United at this point.
Thanks for any wisdom anyone can impart!