Originally Posted by
florens
How? Taking JFK-LHR for example, YQ for WTP is $125 and YQ for CW is $478. So will I have to pay $478-125= $353 when UuA WTP>CW JFK-LHR

That is correct if you had a one way booking.
If your booking is JFK-LHR-JFK and you are in WTP but you only want to upgrade the outbound then what you pay is
Cost = [YQ of JFK-LHR-JFK out CW in WTP] - [YQ of JFK-LHR-JFK all WTP]