why would you be doing this anyways? are you doing this one-way or roundtrip? Are you doing this at airport or arrange in advance?

if you are doing this as one-way, at airport, the applicable full fare one-way J is outrageously expensive... if you are serious about changing your Y to J cuz you won a lottery you could be cheaper off buying a brand-new ticket.... anyways i don't know why anyone buys K and subsequently use cash to upgrade to J cabin (employer paid flight?) just be wary the cost is not as clear cut.