If your booking is for a transit in HKG, changing that to stopover will mean an increase in taxes at a minimum. (However, if you book with stop-over in HKG, this can be avoided as the additional taxes were already included).

If the flights are married segments or on a promotional fare, changes can result in a substantially higher fare.

If you already ticketed, you may have to pay change fee or re-ticketing fee on top of the above.