I am looking to book flights in Business and there is availability for direct miles booking. However when I look at the economy fare it shows the amount the cost would be to upgrade with miles but says there is no availability at this time. Do Emirates have different availability for upgrades to Business than to book directly with miles into business?

Also is it possible when there is availability to book with cash and upgrade with miles as you do the booking or do you have to book the cash fare first and then upgrade?

Thanks