I think this thread demonstrates the stress that is created by the airlines' increasingly complicated passenger choices. The average passenger thinks selecting a seat means they reserved that EXACT seat when, in reality, they are just indicating a seat preference. This misconception is understandable because everyone is not going to read the fine print or CoC. But nowadays, it seems every misunderstanding gets amplified into a hysterical "I've been done wrong and the whole trip was ruined!". I'm not sure what UA can do but add a little pop up at seat selection that the seats are only an indicated preference (which happens to stick probably more than 95% of the time) and are subject to change. In the case of these seat changes, UA should do its best to communicate these changes before the boarding scanner. Implementation can be tough because the boarding area can be a pretty hectic place - accommodating seat changes, processing upgrades and stand-bys, checking BE bags, checking in flight crews, etc