Yep - this sucks United.....!
I was in an aisle E+ on a 2-5-2 777. It then went to a 3-3-3 777 a few days ago - reassigned to an aisle E+. Check in last night, all is good. Check flight status today, back to a 2-5-2, and I'm now in a middle, no aisles available.
So it appears, changes one way are made to similar seats, but the other way around many people are probably getting screwed.