Originally Posted by HKGglobaltrotter

I could be wrong so please correct me if I am. As you issued ex-TPE tickets, you will need to check I class availability from TPE point of sales and not I class availability on HKG - BNE as a standalone flight. Outport tickets have very limited I class fares especially from origins where the ticket prices are significantly lower (such as MNL and TPE for example) so even HKG - BNE has I class selling elsewhere but if ex-TPE I class has been sold out then your bookable upgrade request will not be accepted. It is frustrating for sure!
My advice would be that you always ask the agent to check bookable upgrade availability first before you change the underlying revenue ticket, then have him/her to change your flight and manually process your bookable upgrade request via the phone. They can hold whilst you settle for the change fee.