Cathay will reconpensate those who has been inconvenienced when its due. I find that it's impolite to ask and demand for more.

However, you may simply write into Customer Service and perhaps highlight the fact that you have been 'downgraded' to Business Class when you have a First Class ticket. And see what transpires.

I was recently unfortunate enough myself to have had to fly HKG-AKL on one of the 2 class aircraft when I had a First Class RTW ticket. I hadn't written in about it.