Years ago, AA used to allow you to purchase an upgrade to business class from an award coach seat by paying the difference between full fare coach and full fare business. (In 1989, before AA had any European flights out of Boston, a Boston ticket agent miscalculated this difference and upgraded me on a JFK-ORY flight from an award Y ticket to C for about $150 RT.) Are there any airlines that will still allow you to do this? Nonetheless, given the difference in cost between international Y and international C, even if this were allowed I think that for most routes one would be better off purchasing a discounted Y ticket and then using miles to upgrade to C, rather than using miles for Y and paying to upgrade to C.