I tried. Three times. United said no, all 3 times. And I'm 1K.
I was on my own PNR on a regular economy ticket (K fare) sitting in E+. Companion was on their own PNR in basic economy (N fare). Called the 1K desk three different times. They wouldn't assign a seat to my companion, let alone move them to E+ with me.
If someone is on a basic economy ticket, they aren't allowed to complain. You need to pay roughly $30-70 more dollars to complain!