As above - I have an existing BAH booking. Balance is not due until next July. If I upgrade flights with avios, I assume they are deducted immediately. But what about the cash surcharge element of the upgrade - is that just added to the holiday balance, or does it need to be paid upfront?

If at all relevant, the deposit for the booking was 'paid' using evouchers from prior cancelled bookings, and their combined value was significantly more than the required minimum deposit for the holiday.

thanks
RC