It appears that either the agent said more than he needed to, that he was confused or that OP is confused as to what was said, or all of the above.

Ticket reissue has nothing to do with the additional taxes due. If there is no CC on file, there would have been an add/collect at the airport or the agent would have asked for payment at the time.

But, this situation is vastly different from OpUp's where the carrier places a passenger in a higher class of service for the operational convenience of the carrier. In this instance, the passenger sought to upgrade and supported payment of the upgrade with a GUC.

Same issue arises in the UK with the APD. OPUP means no premium APD, but paid UG, including GUC, does.