Walk away and laughing by sitting in a middle seat? No thanks if this had been my situation.

The seat swap could have been due to change in equipment. Agreeing with Often1 - I would recommend monitoring seating map like a hawk prior to departure; often times a better seat (or in this case, a window E+) would open up due to variety of reasons. If nothing opens up, I would consider a E- window (or aisle), request for refund after the flight, and write to UA.