So, BA have now agreed with me that there is no change fee so that at least proves my point, and for all of us in the future, that there is NO change fee due for changing carrier or date (as long as the ticketed points remain the same)

It is also my understanding that there is no need to re-calculate the taxes as all taxes have been paid previously and I am just revalidating the ticket (otherwise there would ALWAYS be a difference due to exchange rate differences from time of booking to now even if the taxes (in local currency) remained the same - which is quite likely for most tickets)

Lets see what the fares desk come back with and if they just do the change at no cost as they did last time when changing my AKL - JFK (date change from May to June and carrier change from QF to AA Codeshare)