I am Skywards Gold. I don't have quite enough miles to book an F flight to where I want to go and so I am planning to book in J and then use miles to upgrade to F. There is availability for F flights with miles but I can't see availability for upgrade without putting in a booking reference (which I do not have yet). Is it safe to assume that if I could book F directly using miles, the same flights are available for upgrade from J to F? Or is there another way to check before I buy the J flight?