BA are insisting that if I change the date on a companion voucher booking, I will need to pay an extra £270 per person in taxes (plus the £35 change fee).

If I were to make the booking separately on the same dates, I would be charged my original rate of tax and not the extra £270 per person.

They are saying that when you book, you are offered a lower rate of taxes but when you change, you have to pay the higher rate.

Is this correct? Never heard of this before and have changed many companion voucher flights.