Can you see if you can book the particular leg in business? See if there are even open seats on this flight.
Just so I am on the same page. You booked this in business and have been downgraded or you booked this in economy and didn't realize until later? I know you booked in J but when booking did you secure a business class seat? I know you say Downgrade on One Segment in the title, I just want to be sure.