I just had a similar experience when I tried to upgrade GVA-LHR-JFK return BA flights from WT to WT+ using GUF2. The agent said I needed to pay around $250 in additional 'taxes' per ticket - which I declined. What I am wondering is the amount - when I quote the return and compare the normal fares on the website, the difference in taxes and surcharges only comes to about $60. ADP does not apply here as transit is <24h. Is it worth calling again and trying with a different agent ?