Originally Posted by
alim123
... There was no aircraft change. ...
To be sure about this you will need to check the detailed flight history. Changes can occur and then be reversed can create this. Sometimes some one makes an error. Additionally changes to a different tail number of the same aircraft type can cause this problem. Thread wiki provides addition possibilities
Originally Posted by
alim123
...It was obvious that someone with a higher status than me who is traveling in two called up United and persuaded them to move me because they wanted to sit together in an exit row. ...
Very unlikely. Even GS elites have reported they are total this is not possible (against policy).
Originally Posted by
alim123
...Is there somewhere in the T&C that says they reserve the right to do this? ...
Actually yes, seat assignments are considered a preference but are not guaranteed.