I had a similar experience when I purchased a recent ticket on the UA website. When they tried to charge me for E+ seats, I just canceled my seat selections and let the website automatically assign seats at the end. I did not pick an E- seat. After I finished the purchase, my seats were automatically assigned and I was put into E+ and placed on the upgrade list.

Now if I go to try to change my seat, they still try to charge even though I'm trying to make an E+ to an E+ change. Oh well. While not as convenient, a phone call will get the job done.