Actually, Downgrades were available on TG.
I caught a flight out of Shanghai in 2002, to BKK, there were two guys in their business suits all smart and the like, with I "think" economy tickets, but again the flight had been overbooked.
They were forced to pay business class upgrades to get on the flight, with no seats left in Business, so they were made to sit in economy...their business class meals and drinks were brought to them in economy class...they were fucking livid, but I guess they needed to get out at a certain time, it was hilarious watching the stewadresses bringing them wine and "china plates" and real (not plastic) cutlery, while all around were eating the usual crap!