What probably actually happened is that your ticket wasn’t properly reissued. Frighteningly common with GUCs; when using them, always make sure you have a ticket receipt reflecting the right class of service a few days before the flight.
Extra collections of fees are standard for upgrade certificates, although I don’t recall being asked to pay anything extra on past trips to HKG using GUCs (but may not have bothered remembering if it was HK$30

)