I fly United a lot and get silver status almost every year. I've had them occasionally re-assign me to a middle seat after I reserved a specific aisle seat at purchase. United does the same crap as the rest of the airlines.
I fly United twice a month and the only time I had my seat moved in 20 years was when a 15 year old ended up in the exit row and they had him swap with me. I call BS on your story.