November 18 traveling ORD-CVG in "S" Class. I cleared the CPU upgrade list while waiting at the gate at about T -20 and the UA AGENT issued me a new FC Boarding Pass. A few minutes later the E-175 went 'mechanical' and the gate agent asked for "Volunteers" to downgrade as the new plane (E-170) had 6 fewer FC seats. I went to the podium to simply trade my FC boarding pass (as it cost me nothing) for my old seat. Much to my surprise the GA gave me a $300 ETC. I don't know which was more remarkable, getting a CPU as a Platinum or getting a $300 ETC.