While some things should be more automated, and that may eventually happen, in the short-term one often has to be more proactive. You mentioned several times that you called United, but you never indicated if you tried discussing your reassigned seat situation with a GA. From the overheard remark you provided about the person getting E+ without paying, it seems the GAs were reassigning seats to accommodate the aircraft change. Did you check in online and if so, did it show your original seat assignment in E+?