I would like to provide a follow-up for other passengers info.

I called UA Hong Kong and was told I need to go through the TA.

I called UA US and was told that I could proceed with the upgrade. The cost was USD190 vs USD159 offered online. As I do not have a US based credit card, they needed to convert it into HKD. This process took 50 minutes. At the end, the agent told me that the upgrade was successful but as their system is upgrading they were unable to issue the ticket. I was told it would be sent within the hour (this was at about 11pm the day before travel). I gave the agent my number and ask them to call me with any issues.

No call came. I check the UA App and it showed I was in Business Class (but did not check-in). I arrive at the Airport and this is where chaos ensues. Check-in agent sees my ticket as not issued, I explain what happened the previous night. She calls ticketing/ reservations and we are on the phone for 35 minutes, previous transaction was not successful. They are repeatedly stating that they can only accept US credit cards. I am then told that they accept all credit cards apart from HK cards as their is a issue in USD: HKD conversion. After another 15 minutes I get a message from my bank to state the transaction was successful, but took another 15 minutes to show up on UA systems. In total I was at Check-in for close to 1 hour (poor agent!).