Not sure if this is a gotcha or a HUACA.
I was on the phone to redeem a GUF2 on a Miami trip in March, booked in J on an Amex companion voucher, which showed A class availability in Expertflyer (thanks @Karfa for that steer). Very helpful agent on the Gold line until right at the end when he said that the upgrade would cost an extra £423 each in taxes, above the taxes already paid.

Given the text below from the wiki I was not expecting such a tax increase. Have things changed, or is it agent error?

---------------
CW to F
For an upgrade from CW to F there is no difference in carrier surcharge, fees, or APD. Any cash charge should be zero or minimal.

Update: as of summer 2018 it seems carrier surcharge on exUS bookings is now about ~$200 more per flight when going from CW to F so if you are applying a GUF to an exUS booking you should expect to pay this as well. Just to be clear, by exUS bookings I mean those starting in the US like one way JFK-LHR or a return LAX-LHR-LAX. If you book an exUK/EU flight which includes a return from the US you should be paying the UK/EU based carrier surcharge which so far seems to be the same in CW and F, so for a LHR-LAX-LHR return whether you GUF the outnbound/inbound/both flights from CW to F you shouldn't expect to pay more carrier surcharge.
---------------