To give you a rough idea of fares...

Full F class oneway fare between TPE and SYD via HKG = US$1694
Full F class oneway fare between HKG and SYD = US$2706

So if you paid say, US$1650 for your TPE-SYD return in "I" class, and IF HKG says that it's ok for you to upgrade.. then you'd end up paying around US$1880 just for the oneway upgrade from HKG! That's enough for another ticket from TPE!

That is why fares are CHEAPER ex-other places than HKG on CX!

If you really, really want to fly F class, then maybe you might have to buy a discounted F class fare from TPE.. it may cost you less for a return in F than it is for you to upgrade one sector. Generally you cannot combine an "I" class discounted Business with a discounted F class fare.