I had a similar situation in the past on pmUA - vacation plans and work plans collided. The pmUA agent actually suggested that I simply not use the return portion of my vacation ticket and I booked a new ticket originating in the city that I was vacationing. The minor difference (or possibly major difference from the UA point of view) was that this was simply not flying the return portion of a RT, which may or may not have changed the original revenue that UA got from me.
The bottom line IME is that most airline employees can understand when this occurs due to circumstances beyond your control as opposed to a deliberate attempt to game the system. As for the morality discussion, I draw the line at intentionally doing hidden city ticketing vs having your plans change.