Spending miles in increments
I'd like to upgrade a ZRH-DXB-DAC trip. 50k miles to upgrade. DXB-DAC is 30k, ZRH-DXB is 32500. The problem is, ZRH-DXB is currently not available.
What happens if ZRH-DXB becomes available after I upgrade DXB-DAC? Do I pay the difference to 50k miles or will they charge me 32500? I don't want to pay 12500 miles extra, but I also want to book in case availability disappears.